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8.6. PostgreSQL Extensions to the JDBC API

Postgres is an extensible database system. You can add your own functions to the backend, which can then be called from queries, or even add your own data types. As these are facilities unique to Postgres, we support them from Java, with a set of extension API's. Some features within the core of the standard driver actually use these extensions to implement Large Objects, etc.

8.6.1. Accessing the Extensions

To access some of the extensions, you need to use some extra methods in the org.postgresql.Connection class. In this case, you would need to case the return value of Driver.getConnection(). For example:

Connection db = Driver.getConnection(url, username, password);
// ...
// later on
Fastpath fp = ((org.postgresql.Connection)db).getFastpathAPI();

8.6.1.1. Class org.postgresql.Connection

public class Connection extends Object implements Connection

java.lang.Object
   |
   +----org.postgresql.Connection

These are the extra methods used to gain access to PostgreSQL's extensions. Methods defined by java.sql.Connection are not listed.

8.6.1.1.1. Methods

  • public Fastpath getFastpathAPI() throws SQLException
    

    This returns the Fastpath API for the current connection. It is primarily used by the Large Object API.

    The best way to use this is as follows:

    import org.postgresql.fastpath.*;
    ...
    Fastpath fp = ((org.postgresql.Connection)myconn).getFastpathAPI();
    
    where myconn is an open Connection to PostgreSQL.

    Returns: Fastpath object allowing access to functions on the PostgreSQL backend.

    Throws: SQLException by Fastpath when initializing for first time

  • public LargeObjectManager getLargeObjectAPI() throws SQLException
    
    This returns the Large Object API for the current connection.

    The best way to use this is as follows:

    import org.postgresql.largeobject.*;
    ...
    LargeObjectManager lo = ((org.postgresql.Connection)myconn).getLargeObjectAPI();
    
    where myconn is an open Connection to PostgreSQL.

    Returns: LargeObject object that implements the API

    Throws: SQLException by LargeObject when initializing for first time

  • public void addDataType(String type, String name)
    
    This allows client code to add a handler for one of PostgreSQL's more unique data types. Normally, a data type not known by the driver is returned by ResultSet.getObject() as a PGobject instance. This method allows you to write a class that extends PGobject, and tell the driver the type name, and class name to use. The down side to this, is that you must call this method each time a connection is made.

    The best way to use this is as follows:

     ...
    ((org.postgresql.Connection)myconn).addDataType("mytype","my.class.name");
     ...
    
    where myconn is an open Connection to PostgreSQL. The handling class must extend org.postgresql.util.PGobject.

8.6.1.2. Class org.postgresql.Fastpath

public class Fastpath extends Object

java.lang.Object
   |
   +----org.postgresql.fastpath.Fastpath

Fastpath is an API that exists within the libpq C interface, and allows a client machine to execute a function on the database backend. Most client code will not need to use this method, but it is provided because the Large Object API uses it.

To use, you need to import the org.postgresql.fastpath package, using the line:

import org.postgresql.fastpath.*;
Then, in your code, you need to get a FastPath object:
Fastpath fp = ((org.postgresql.Connection)conn).getFastpathAPI();
This will return an instance associated with the database connection that you can use to issue commands. The casing of Connection to org.postgresql.Connection is required, as the getFastpathAPI() is an extension method, not part of JDBC. Once you have a Fastpath instance, you can use the fastpath() methods to execute a backend function.

See Also: FastpathFastpathArg, LargeObject

8.6.1.2.1. Methods

  • public Object fastpath(int fnid,
                           boolean resulttype,
                           FastpathArg args[]) throws SQLException
    

    Send a function call to the PostgreSQL backend.

    Parameters: fnid - Function id resulttype - True if the result is an integer, false for other results args - FastpathArguments to pass to fastpath

    Returns: null if no data, Integer if an integer result, or byte[] otherwise

  • public Object fastpath(String name,
                           boolean resulttype,
                           FastpathArg args[]) throws SQLException
    

    Send a function call to the PostgreSQL backend by name.

    Note: The mapping for the procedure name to function id needs to exist, usually to an earlier call to addfunction(). This is the preferred method to call, as function id's can/may change between versions of the backend. For an example of how this works, refer to org.postgresql.LargeObject

    Parameters: name - Function name resulttype - True if the result is an integer, false for other results args - FastpathArguments to pass to fastpath

    Returns: null if no data, Integer if an integer result, or byte[] otherwise

    See Also: LargeObject

  •           
    public int getInteger(String name,
                          FastpathArg args[]) throws SQLException
    

    This convenience method assumes that the return value is an Integer

    Parameters: name - Function name args - Function arguments

    Returns: integer result

    Throws: SQLException if a database-access error occurs or no result

  • public byte[] getData(String name,
                          FastpathArg args[]) throws SQLException
    

    This convenience method assumes that the return value is binary data.

    Parameters: name - Function name args - Function arguments

    Returns: byte[] array containing result

    Throws: SQLException if a database-access error occurs or no result

  • public void addFunction(String name,
                            int fnid)
    

    This adds a function to our look-up table. User code should use the addFunctions method, which is based upon a query, rather than hard coding the oid. The oid for a function is not guaranteed to remain static, even on different servers of the same version.

  • public void addFunctions(ResultSet rs) throws SQLException
    

    This takes a ResultSet containing two columns. Column 1 contains the function name, Column 2 the oid. It reads the entire ResultSet, loading the values into the function table.

    Important: Remember to close() the ResultSet after calling this!

    Implementation note about function name look-ups: PostgreSQL stores the function id's and their corresponding names in the pg_proc table. To speed things up locally, instead of querying each function from that table when required, a Hashtable is used. Also, only the function's required are entered into this table, keeping connection times as fast as possible.

    The org.postgresql.LargeObject class performs a query upon its start-up, and passes the returned ResultSet to the addFunctions() method here. Once this has been done, the Large Object API refers to the functions by name.

    Do not think that manually converting them to the oid's will work. Okay, they will for now, but they can change during development (there was some discussion about this for V7.0), so this is implemented to prevent any unwarranted headaches in the future.

    See Also: LargeObjectManager

  • public int getID(String name) throws SQLException
    

    This returns the function id associated by its name If addFunction() or addFunctions() have not been called for this name, then an SQLException is thrown.

8.6.1.3. Class org.postgresql.fastpath.FastpathArg

public class FastpathArg extends Object

java.lang.Object
   |
   +----org.postgresql.fastpath.FastpathArg

Each fastpath call requires an array of arguments, the number and type dependent on the function being called. This class implements methods needed to provide this capability.

For an example on how to use this, refer to the org.postgresql.LargeObject package.

See Also: Fastpath, LargeObjectManager, LargeObject

8.6.1.3.1. Constructors

  • public FastpathArg(int value)
    

    Constructs an argument that consists of an integer value

    Parameters: value - int value to set

  • public FastpathArg(byte bytes[])
    

    Constructs an argument that consists of an array of bytes

    Parameters: bytes - array to store

  • public FastpathArg(byte buf[],
                       int off,
                       int len)
    

    Constructs an argument that consists of part of a byte array

    Parameters:

    buf

    source array

    off

    offset within array

    len

    length of data to include

  • public FastpathArg(String s)
    

    Constructs an argument that consists of a String.

8.6.2. Geometric Data Types

PostgreSQL has a set of data types that can store geometric features into a table. These include single points, lines, and polygons. We support these types in Java with the org.postgresql.geometric package. It contains classes that extend the org.postgresql.util.PGobject class. Refer to that class for details on how to implement your own data type handlers.

Class org.postgresql.geometric.PGbox

java.lang.Object
   |
   +----org.postgresql.util.PGobject
           |
           +----org.postgresql.geometric.PGbox

   public class PGbox extends PGobject implements Serializable, 
Cloneable

   This represents the box data type within PostgreSQL.

Variables

 public PGpoint point[]

          These are the two corner points of the box.

Constructors

 public PGbox(double x1,
              double y1,
              double x2,
              double y2)

        Parameters:
                x1 - first x coordinate
                y1 - first y coordinate
                x2 - second x coordinate
                y2 - second y coordinate

 public PGbox(PGpoint p1,
              PGpoint p2)

        Parameters:
                p1 - first point
                p2 - second point

 public PGbox(String s) throws SQLException
                            
        Parameters:
                s - Box definition in PostgreSQL syntax

        Throws: SQLException
                if definition is invalid
                
 public PGbox()

          Required constructor
              
Methods

 public void setValue(String value) throws SQLException
                
          This method sets the value of this object. It should be 
overridden, but still called by subclasses.
                            
        Parameters:
                value - a string representation of the value of the 
object
        Throws: SQLException
                thrown if value is invalid for this type

        Overrides:
                setValue in class PGobject

 public boolean equals(Object obj)

        Parameters:
                obj - Object to compare with
                
        Returns:
                true if the two boxes are identical
          
        Overrides:
                equals in class PGobject

 public Object clone()
        
          This must be overridden to allow the object to be cloned

        Overrides:
                clone in class PGobject
   
 public String getValue()
        
        Returns:
                the PGbox in the syntax expected by PostgreSQL

        Overrides:
                getValue in class PGobject

Class org.postgresql.geometric.PGcircle

java.lang.Object
   |
   +----org.postgresql.util.PGobject
           |
           +----org.postgresql.geometric.PGcircle
        
   public class PGcircle extends PGobject implements Serializable, 
Cloneable
               
   This represents PostgreSQL's circle data type, consisting of a point 
and a radius

Variables

 public PGpoint center
           
          This is the center point
 
public double radius
           
          This is the radius
   
Constructors   

 public PGcircle(double x,
                 double y,
                 double r)
          
        Parameters:
               x - coordinate of center
                y - coordinate of center
                r - radius of circle

 public PGcircle(PGpoint c,
                 double r)
          
        Parameters:
                c - PGpoint describing the circle's center
                r - radius of circle

 public PGcircle(String s) throws SQLException

        Parameters:
                s - definition of the circle in PostgreSQL's syntax.

        Throws: SQLException
                on conversion failure

 public PGcircle()

          This constructor is used by the driver.
            
Methods   

 public void setValue(String s) throws SQLException

        Parameters:
                s - definition of the circle in PostgreSQL's syntax.

        Throws: SQLException
                on conversion failure

        Overrides:
                setValue in class PGobject

 public boolean equals(Object obj)

        Parameters:
                obj - Object to compare with
            
        Returns:
                true if the two boxes are identical

        Overrides:
                equals in class PGobject

 public Object clone()

          This must be overridden to allow the object to be cloned

        Overrides:
                clone in class PGobject

 public String getValue()

        Returns:
                the PGcircle in the syntax expected by PostgreSQL
        
        Overrides:
                getValue in class PGobject

Class org.postgresql.geometric.PGline

java.lang.Object
   |
   +----org.postgresql.util.PGobject
           |
           +----org.postgresql.geometric.PGline

   public class PGline extends PGobject implements Serializable, 
Cloneable

   This implements a line consisting of two points. Currently line is 
not yet implemented in the backend, but this class ensures that when 
it's done were ready for it.

Variables
   
 public PGpoint point[]
     
          These are the two points.

Constructors

 public PGline(double x1,
               double y1,
               double x2,
               double y2)

        Parameters:
                x1 - coordinate for first point
                y1 - coordinate for first point
                x2 - coordinate for second point
                y2 - coordinate for second point

 public PGline(PGpoint p1,
               PGpoint p2)
     
        Parameters:
                p1 - first point
                p2 - second point

 public PGline(String s) throws SQLException
               
        Parameters:
                s - definition of the circle in PostgreSQL's syntax.

        Throws: SQLException
                on conversion failure

 public PGline()

          required by the driver
               
Methods

 public void setValue(String s) throws SQLException

        Parameters:
                s - Definition of the line segment in PostgreSQL's 
syntax

        Throws: SQLException
                on conversion failure

        Overrides:
                setValue in class PGobject
                
 public boolean equals(Object obj)

        Parameters:
                obj - Object to compare with
               
        Returns:
                true if the two boxes are identical
   
        Overrides:
                equals in class PGobject

 public Object clone()
        
          This must be overridden to allow the object to be cloned

        Overrides:
                clone in class PGobject

 public String getValue()
   
        Returns:
                the PGline in the syntax expected by PostgreSQL
        
        Overrides:
                getValue in class PGobject

Class org.postgresql.geometric.PGlseg
             
java.lang.Object
   |
   +----org.postgresql.util.PGobject
           |
           +----org.postgresql.geometric.PGlseg
          
   public class PGlseg extends PGobject implements Serializable, 
Cloneable
 
   This implements a lseg (line segment) consisting of two points

Variables

 public PGpoint point[]
           
          These are the two points.

Constructors
   
 public PGlseg(double x1,
               double y1,
               double x2,
               double y2)
     
        Parameters:

                x1 - coordinate for first point
                y1 - coordinate for first point
                x2 - coordinate for second point
                y2 - coordinate for second point

 public PGlseg(PGpoint p1,
               PGpoint p2)
           
        Parameters:
                p1 - first point
                p2 - second point
   
 public PGlseg(String s) throws SQLException

        Parameters:
                s - definition of the circle in PostgreSQL's syntax.

        Throws: SQLException
                on conversion failure

 public PGlseg()

          required by the driver
               
Methods    
   
 public void setValue(String s) throws SQLException
   
        Parameters:
                s - Definition of the line segment in PostgreSQL's 
syntax

        Throws: SQLException
                on conversion failure
     
        Overrides:
                setValue in class PGobject
                
 public boolean equals(Object obj)

        Parameters:
                obj - Object to compare with
               
        Returns:
                true if the two boxes are identical
   
        Overrides:
                equals in class PGobject
   
 public Object clone()

          This must be overridden to allow the object to be cloned

        Overrides:
               clone in class PGobject

 public String getValue()

        Returns:
                the PGlseg in the syntax expected by PostgreSQL
        
        Overrides:
                getValue in class PGobject

Class org.postgresql.geometric.PGpath
                                
java.lang.Object
   |
   +----org.postgresql.util.PGobject
           |
           +----org.postgresql.geometric.PGpath
          
   public class PGpath extends PGobject implements Serializable, 
Cloneable
               
   This implements a path (a multiply segmented line, which may be 
closed)
           
Variables

 public boolean open
               
          True if the path is open, false if closed

 public PGpoint points[]

          The points defining this path

Constructors   

 public PGpath(PGpoint points[],
               boolean open)
          
        Parameters:
                points - the PGpoints that define the path
                open - True if the path is open, false if closed

 public PGpath()

          Required by the driver

 public PGpath(String s) throws SQLException

        Parameters:
                s - definition of the circle in PostgreSQL's syntax.

        Throws: SQLException
                on conversion failure

Methods

 public void setValue(String s) throws SQLException
   
        Parameters:
                s - Definition of the path in PostgreSQL's syntax
           
        Throws: SQLException
                on conversion failure

        Overrides:
                setValue in class PGobject

 public boolean equals(Object obj)

        Parameters:
                obj - Object to compare with

        Returns:
                true if the two boxes are identical

        Overrides:
                equals in class PGobject

 public Object clone()

          This must be overridden to allow the object to be cloned

        Overrides:
                clone in class PGobject

 public String getValue()

          This returns the polygon in the syntax expected by 
PostgreSQL

        Overrides:
                getValue in class PGobject

 public boolean isOpen()

     This returns true if the path is open

 public boolean isClosed()

     This returns true if the path is closed

 public void closePath()

     Marks the path as closed

 public void openPath()

     Marks the path as open

Class org.postgresql.geometric.PGpoint
                                
java.lang.Object
   |
   +----org.postgresql.util.PGobject
           |
           +----org.postgresql.geometric.PGpoint
          
   public class PGpoint extends PGobject implements Serializable, 
Cloneable

   This implements a version of java.awt.Point, except it uses double 
to represent the coordinates.

   It maps to the point data type in PostgreSQL.

Variables

 public double x

          The X coordinate of the point

 public double y

          The Y coordinate of the point

Constructors

 public PGpoint(double x,
                double y)

        Parameters:
                x - coordinate
                y - coordinate

 public PGpoint(String value) throws SQLException
     
          This is called mainly from the other geometric types, when a 
point is embedded within their definition.
             
        Parameters:
                value - Definition of this point in PostgreSQL's 
syntax
   
 public PGpoint()
          
          Required by the driver

Methods

 public void setValue(String s) throws SQLException

        Parameters:
                s - Definition of this point in PostgreSQL's syntax

        Throws: SQLException
                on conversion failure

        Overrides:
                setValue in class PGobject
          
 public boolean equals(Object obj)

        Parameters:
                obj - Object to compare with

        Returns:
                true if the two boxes are identical

        Overrides:
                equals in class PGobject

 public Object clone()
                
          This must be overridden to allow the object to be cloned

        Overrides:
                clone in class PGobject
          
 public String getValue()       
    
        Returns:
                the PGpoint in the syntax expected by PostgreSQL

        Overrides:
                getValue in class PGobject
          
 public void translate(int x,
                       int y)

          Translate the point with the supplied amount.

        Parameters:
                x - integer amount to add on the x axis
                y - integer amount to add on the y axis

 public void translate(double x,
                       double y)
          
          Translate the point with the supplied amount.
 
        Parameters:
                x - double amount to add on the x axis
                y - double amount to add on the y axis

 public void move(int x,
                  int y)
                
          Moves the point to the supplied coordinates.

        Parameters:
                x - integer coordinate
                y - integer coordinate

public void move(double x,
                  double y)
          
          Moves the point to the supplied coordinates.

        Parameters:
                x - double coordinate
                y - double coordinate

 public void setLocation(int x,
                         int y)

          Moves the point to the supplied coordinates. refer to
          java.awt.Point for description of this

        Parameters:
                x - integer coordinate
                y - integer coordinate

        See Also:
                Point

 public void setLocation(Point p)

          Moves the point to the supplied java.awt.Point refer to
          java.awt.Point for description of this

        Parameters:
                p - Point to move to

        See Also:
                Point

Class org.postgresql.geometric.PGpolygon
                                
java.lang.Object
   |
   +----org.postgresql.util.PGobject
           |
           +----org.postgresql.geometric.PGpolygon

   public class PGpolygon extends PGobject implements Serializable, 
Cloneable
               
   This implements the polygon data type within PostgreSQL.

Variables

 public PGpoint points[]

          The points defining the polygon
                                
Constructors

 public PGpolygon(PGpoint points[])

          Creates a polygon using an array of PGpoints

        Parameters:
                points - the points defining the polygon

 public PGpolygon(String s) throws SQLException
                 
        Parameters:
                s - definition of the circle in PostgreSQL's syntax.

        Throws: SQLException
                on conversion failure

 public PGpolygon()

          Required by the driver

Methods

 public void setValue(String s) throws SQLException

        Parameters:
                s - Definition of the polygon in PostgreSQL's syntax

        Throws: SQLException
                on conversion failure

        Overrides:
                setValue in class PGobject

 public boolean equals(Object obj)
     
        Parameters:
                obj - Object to compare with
                                
        Returns:
                true if the two boxes are identical

        Overrides:
                equals in class PGobject

 public Object clone()
        
          This must be overridden to allow the object to be cloned

        Overrides:
                clone in class PGobject
                 
 public String getValue()

        Returns:
                the PGpolygon in the syntax expected by PostgreSQL

        Overrides:
                getValue in class PGobject

8.6.3. Large Objects

Large objects are supported in the standard JDBC specification. However, that interface is limited, and the API provided by PostgreSQL allows for random access to the objects contents, as if it was a local file.

The org.postgresql.largeobject package provides to Java the libpq C interface's large object API. It consists of two classes, LargeObjectManager, which deals with creating, opening and deleting large objects, and LargeObject which deals with an individual object.

8.6.3.1. Class org.postgresql.largeobject.LargeObject

public class LargeObject extends Object

java.lang.Object
   |
   +----org.postgresql.largeobject.LargeObject

This class implements the large object interface to PostgreSQL.

It provides the basic methods required to run the interface, plus a pair of methods that provide InputStream and OutputStream classes for this object.

Normally, client code would use the getAsciiStream, getBinaryStream, or getUnicodeStream methods in ResultSet, or setAsciiStream, setBinaryStream, or setUnicodeStream methods in PreparedStatement to access Large Objects.

However, sometimes lower level access to Large Objects are required, that are not supported by the JDBC specification.

Refer to org.postgresql.largeobject.LargeObjectManager on how to gain access to a Large Object, or how to create one.

See Also: LargeObjectManager

8.6.3.1.1. Variables

public static final int SEEK_SET

Indicates a seek from the beginning of a file

public static final int SEEK_CUR

Indicates a seek from the current position

public static final int SEEK_END

Indicates a seek from the end of a file

8.6.3.1.2. Methods

  • public int getOID()
    

    Returns the OID of this LargeObject

  • public void close() throws SQLException
    

    This method closes the object. You must not call methods in this object after this is called.

  • public byte[] read(int len) throws SQLException
    

    Reads some data from the object, and return as a byte[] array

  • public void read(byte buf[],
                     int off,
                     int len) throws SQLException
    

    Reads some data from the object into an existing array

    Parameters:

    buf

    destination array

    off

    offset within array

    len

    number of bytes to read

  • public void write(byte buf[]) throws SQLException
    

    Writes an array to the object

  • public void write(byte buf[],
                      int off,
                      int len) throws SQLException
    

    Writes some data from an array to the object

    Parameters:

    buf

    destination array

    off

    offset within array

    len

    number of bytes to write

8.6.3.2. Class org.postgresql.largeobject.LargeObjectManager

                                
public class LargeObjectManager extends Object

java.lang.Object
   |
   +----org.postgresql.largeobject.LargeObjectManager

This class implements the large object interface to PostgreSQL. It provides methods that allow client code to create, open and delete large objects from the database. When opening an object, an instance of org.postgresql.largeobject.LargeObject is returned, and its methods then allow access to the object.

This class can only be created by org.postgresql.Connection. To get access to this class, use the following segment of code:

import org.postgresql.largeobject.*;
Connection  conn;
LargeObjectManager lobj;
// ... code that opens a connection ...
lobj = ((org.postgresql.Connection)myconn).getLargeObjectAPI();

Normally, client code would use the getAsciiStream, getBinaryStream, or getUnicodeStream methods in ResultSet, or setAsciiStream, setBinaryStream, or setUnicodeStream methods in PreparedStatement to access Large Objects. However, sometimes lower level access to Large Objects are required, that are not supported by the JDBC specification.

Refer to org.postgresql.largeobject.LargeObject on how to manipulate the contents of a Large Object.

8.6.3.2.1. Variables

public static final int WRITE

This mode indicates we want to write to an object.

public static final int READ

This mode indicates we want to read an object.

public static final int READWRITE

This mode is the default. It indicates we want read and write access to a large object.

8.6.3.2.2. Methods

  • public LargeObject open(int oid) throws SQLException
    

    This opens an existing large object, based on its OID. This method assumes that READ and WRITE access is required (the default).

  • public LargeObject open(int oid,
                            int mode) throws SQLException
    

    This opens an existing large object, based on its OID, and allows setting the access mode.

  • public int create() throws SQLException
    

    This creates a large object, returning its OID. It defaults to READWRITE for the new object's attributes.

  • public int create(int mode) throws SQLException
    

    This creates a large object, returning its OID, and sets the access mode.

  • public void delete(int oid) throws SQLException
    

    This deletes a large object.

  • public void unlink(int oid) throws SQLException
    

    This deletes a large object. It is identical to the delete method, and is supplied as the C API uses "unlink".

8.6.4. Object Serialization

PostgreSQL is not a normal SQL database. It is more extensible than most other databases, and does support object oriented features that are unique to it.

One of the consequences of this, is that you can have one table refer to a row in another table. For example:

test=> create table users (username name,fullname text);
CREATE
test=> create table server (servername name,adminuser users);
CREATE
test=> insert into users values ('peter','Peter Mount');
INSERT 2610132 1
test=> insert into server values ('maidast',2610132::users);
INSERT 2610133 1
test=> select * from users;
username|fullname      
--------+--------------
peter   |Peter Mount   
(1 row)

test=> select * from server;
servername|adminuser
----------+---------
maidast   |  2610132
(1 row)
Okay, the above example shows that we can use a table name as a field, and the row's oid value is stored in that field.

What does this have to do with Java?

In Java, you can store an object to a Stream as long as it's class implements the java.io.Serializable interface. This process, known as Object Serialization, can be used to store complex objects into the database.

Now, under JDBC, you would have to use a Large Object to store them. However, you cannot perform queries on those objects.

What the org.postgresql.util.Serialize class does, is provide a means of storing an object as a table, and to retrieve that object from a table. In most cases, you would not need to access this class direct, but you would use the PreparedStatement.setObject() and ResultSet.getObject() methods. Those methods will check the objects class name against the table's in the database. If a match is found, it assumes that the object is a Serialized object, and retrieves it from that table. As it does so, if the object contains other serialized objects, then it recurses down the tree.

Sound's complicated? In fact, it's simpler than what I wrote - it's just difficult to explain.

The only time you would access this class, is to use the create() methods. These are not used by the driver, but issue one or more "create table" statements to the database, based on a Java Object or Class that you want to serialize.

Oh, one last thing. If your object contains a line like:

public int oid;
then, when the object is retrieved from the table, it is set to the oid within the table. Then, if the object is modified, and re- serialized, the existing entry is updated.

If the oid variable is not present, then when the object is serialized, it is always inserted into the table, and any existing entry in the table is preserved.

Setting oid to 0 before serialization, will also cause the object to be inserted. This enables an object to be duplicated in the database.

Class org.postgresql.util.Serialize

java.lang.Object
   |
   +----org.postgresql.util.Serialize

   public class Serialize extends Object

   This class uses PostgreSQL's object oriented features to store Java 
Objects. It does this by mapping a Java Class name to a table in the 
database. Each entry in this new table then represents a Serialized 
instance of this class. As each entry has an OID (Object IDentifier), 
this OID can be included in another table. This is too complex to show 
here, and will be documented in the main documents in more detail.

Constructors

 public Serialize(Connection c,
                  String type) throws SQLException

          This creates an instance that can be used to serialize 
or deserialize a Java object from a PostgreSQL table.

Methods

 public Object fetch(int oid) throws SQLException

          This fetches an object from a table, given it's OID

        Parameters:
                oid - The oid of the object

        Returns:
                Object relating to oid

        Throws: SQLException
                on error

 public int store(Object o) throws SQLException

          This stores an object into a table, returning it's OID.

          If the object has an int called OID, and it is > 0, then 
that value is used for the OID, and the table will be updated. If the 
value of OID is 0, then a new row will be created, and the value of 
OID will be set in the object. This enables an object's value in the 
database to be updateable. If the object has no int called OID, then 
the object is stored. However if the object is later retrieved, 
amended and stored again, it's new state will be appended to the 
table, and will not overwrite the old entries.

        Parameters:
                o - Object to store (must implement Serializable)

        Returns:
                oid of stored object

        Throws: SQLException
                on error
 
 public static void create(Connection con,
                           Object o) throws SQLException

          This method is not used by the driver, but it creates a 
table, given a Serializable Java Object. It should be used before 
serializing any objects.

        Parameters:
                c - Connection to database
                o - Object to base table on

        Throws: SQLException
                on error

                   Returns:
                Object relating to oid

        Throws: SQLException
                on error

 public int store(Object o) throws SQLException

          This stores an object into a table, returning it's OID.

          If the object has an int called OID, and it is > 0, then 
that value is used for the OID, and the table will be updated. If the 
value of OID is 0, then a new row will be created, and the value of 
OID will be set in the object. This enables an object's value in the 
database to be updateable. If the object has no int called OID, then 
the object is stored. However if the object is later retrieved, 
amended and stored again, it's new state will be appended to the 
table, and will not overwrite the old entries.

        Parameters:
                o - Object to store (must implement Serializable)

        Returns:
                oid of stored object

        Throws: SQLException
                on error
 
 public static void create(Connection con,
                           Object o) throws SQLException

          This method is not used by the driver, but it creates a 
table, given a Serializable Java Object. It should be used before 
serializing any objects.

        Parameters:
                c - Connection to database
                o - Object to base table on

        Throws: SQLException
                on error
                
 public static void create(Connection con,
                           Class c) throws SQLException

          This method is not used by the driver, but it creates a 
table, given a Serializable Java Object. It should be used before 
serializing any objects.

        Parameters:
                c - Connection to database
                o - Class to base table on

        Throws: SQLException
                on error

 public static String toPostgreSQL(String name) throws SQLException
          
          This converts a Java Class name to a PostgreSQL table, by
          replacing . with _

          Because of this, a Class name may not have _ in the name.

          Another limitation, is that the entire class name (including
          packages) cannot be longer than 31 characters (a limit 
forced by PostgreSQL).

        Parameters:
                name - Class name

        Returns:
                PostgreSQL table name

        Throws: SQLException
                on error
          
 public static String toClassName(String name) throws SQLException

          This converts a PostgreSQL table to a Java Class name, by
          replacing _ with .

        Parameters:
                name - PostgreSQL table name
  
        Returns:
                Class name

        Throws: SQLException
                on error
Utility Classes

The org.postgresql.util package contains classes used by the internals of 
the main driver, and the other extensions.

Class org.postgresql.util.PGmoney
                                
java.lang.Object
   |
   +----org.postgresql.util.PGobject
           |
           +----org.postgresql.util.PGmoney

   public class PGmoney extends PGobject implements Serializable, 
Cloneable
               
   This implements a class that handles the PostgreSQL money type

Variables

 public double val
                                
          The value of the field

Constructors
           
 public PGmoney(double value)
   
        Parameters:
                value - of field
               
 public PGmoney(String value) throws SQLException
   
          This is called mainly from the other geometric types, when a 
point is imbeded within their definition.

        Parameters:
                value - Definition of this point in PostgreSQL's 
syntax

 public PGmoney()

          Required by the driver

Methods

 public void setValue(String s) throws SQLException

        Parameters:
                s - Definition of this point in PostgreSQL's syntax

        Throws: SQLException
                on conversion failure

        Overrides:
                setValue in class PGobject

 public boolean equals(Object obj)

        Parameters:
                obj - Object to compare with
                                
        Returns:
                true if the two boxes are identical

        Overrides:
                equals in class PGobject

 public Object clone()
                
          This must be overridden to allow the object to be cloned

        Overrides:
                clone in class PGobject

 public String getValue()

        Returns:
                the PGpoint in the syntax expected by PostgreSQL

        Overrides:
                getValue in class PGobject

Class org.postgresql.util.PGobject

java.lang.Object
   |
   +----org.postgresql.util.PGobject

   public class PGobject extends Object implements Serializable, 
Cloneable
               
   This class is used to describe data types that are unknown by
     JDBC 
Standard.
    A call to org.postgresql.Connection permits a class that extends this 
class to be associated with a named type. This is how the 
org.postgresql.geometric package operates.
    ResultSet.getObject() will return this class for any type that is 
not recognized on having it's own handler. Because of this, any 
PostgreSQL data type is supported.

Constructors

 public PGobject()

          This is called by org.postgresql.Connection.getObject() to 
create the object.

Methods

 public final void setType(String type)

          This method sets the type of this object.

          It should not be extended by subclasses, hence its final

        Parameters:
                type - a string describing the type of the object

 public void setValue(String value) throws SQLException

          This method sets the value of this object. It must be 
overridden.

        Parameters:
                value - a string representation of the value of the
                object

        Throws: SQLException
                thrown if value is invalid for this type
    
 public final String getType()

          As this cannot change during the life of the object, it's 
final.

        Returns:
                the type name of this object

 public String getValue()

          This must be overridden, to return the value of the object, 
in the form required by PostgreSQL.

        Returns:
                the value of this object

 public boolean equals(Object obj)

          This must be overridden to allow comparisons of objects

        Parameters:
                obj - Object to compare with

        Returns:
                true if the two boxes are identical

        Overrides:
                equals in class Object

 public Object clone()

          This must be overridden to allow the object to be cloned

        Overrides:
                clone in class Object

 public String toString()

          This is defined here, so user code need not override it.
          
        Returns:
                the value of this object, in the syntax expected by 
PostgreSQL

        Overrides:
                toString in class Object

Class org.postgresql.util.PGtokenizer

java.lang.Object
   |
   +----org.postgresql.util.PGtokenizer

   public class PGtokenizer extends Object

   This class is used to tokenize the text output of PostgreSQL.

   We could have used StringTokenizer to do this, however, we needed 
to handle nesting of '(' ')' '[' ']' '<' and '>' as these are used by 
the geometric data types.

   It's mainly used by the geometric classes, but is useful in parsing 
any output from custom data types output from PostgreSQL.
                 
   See Also:
          PGbox, PGcircle, PGlseg, PGpath, PGpoint, PGpolygon
          
Constructors

 public PGtokenizer(String string,
                    char delim)

          Create a tokenizer.

        Parameters:
                string - containing tokens
                delim - single character to split the tokens

Methods
        
 public int tokenize(String string,
                     char delim)

          This resets this tokenizer with a new string and/or 
delimiter.

        Parameters:
                string - containing tokens
                delim - single character to split the tokens

 public int getSize()

        Returns:
                the number of tokens available

 public String getToken(int n)

        Parameters:
                n - Token number ( 0 ... getSize()-1 )

        Returns:
                The token value

 public PGtokenizer tokenizeToken(int n,
                                  char delim)

          This returns a new tokenizer based on one of our tokens. The 
geometric data types use this to process nested tokens (usually 
PGpoint).

        Parameters:
                n - Token number ( 0 ... getSize()-1 )
                delim - The delimiter to use

        Returns:
                A new instance of PGtokenizer based on the token

 public static String remove(String s,
                             String l,
                             String t)

          This removes the lead/trailing strings from a string

        Parameters:
                s - Source string
                l - Leading string to remove
                t - Trailing string to remove
                
        Returns:
                String without the lead/trailing strings

 public void remove(String l,
                    String t)

          This removes the lead/trailing strings from all tokens

        Parameters:
                l - Leading string to remove
                t - Trailing string to remove

 public static String removePara(String s)

          Removes ( and ) from the beginning and end of a string

        Parameters:
                s - String to remove from

        Returns:
                String without the ( or )

 public void removePara()

          Removes ( and ) from the beginning and end of all tokens

        Returns:
                String without the ( or )

 public static String removeBox(String s)
   
          Removes [ and ] from the beginning and end of a string

        Parameters:
                s - String to remove from
   
        Returns:
                String without the [ or ]

 public void removeBox()

          Removes [ and ] from the beginning and end of all tokens

        Returns:
                String without the [ or ]

 public static String removeAngle(String s)

          Removes < and > from the beginning and end of a string

        Parameters:
                s - String to remove from

        Returns:
                String without the < or >

 public void removeAngle()

          Removes < and > from the beginning and end of all tokens

        Returns:
                String without the < or >

Class org.postgresql.util.Serialize

This was documented earlier under Object Serialization.

Class org.postgresql.util.UnixCrypt
              
java.lang.Object
   |
   +----org.postgresql.util.UnixCrypt

   public class UnixCrypt extends Object

   This class provides us with the ability to encrypt passwords when 
sent over the network stream

   Contains static methods to encrypt and compare passwords with Unix 
encrypted passwords.

   See John Dumas's Java Crypt page for the original source.

   http://www.zeh.com/local/jfd/crypt.html

Methods

 public static final String crypt(String salt,
                                  String original)

          Encrypt a password given the clear-text password and a 
"salt".
   
        Parameters:
                salt - A two-character string representing the salt 
used
                to iterate the encryption engine in lots of different
                ways. If you are generating a new encryption then this
                value should be randomized.
                original - The password to be encrypted.

        Returns:
                A string consisting of the 2-character salt followed 
by
                the encrypted password.
              
 public static final String crypt(String original)

          Encrypt a password given the clear-text password. This method 
generates a random salt using the 'java.util.Random' class.

        Parameters:
                original - The password to be encrypted.
   
        Returns: 
                A string consisting of the 2-character salt followed 
by
                the encrypted password.
               
 public static final boolean matches(String encryptedPassword,
                                     String enteredPassword)
                 
          Check that enteredPassword encrypts to encryptedPassword.
               
        Parameters:
                encryptedPassword - The encryptedPassword. The first 
two characters are assumed to be the salt. This string would be the 
same as one found in a Unix /etc/passwd file.
                enteredPassword - The password as entered by the user 
(or otherwise acquired).

        Returns:
                true if the password should be considered correct.
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