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.
Now, as these are facilities unique to us, 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.
Accessing the extensions
To access some of the extensions, you need to use some extra methods
in the postgresql.Connection class. In this case, you would need to
case the return value of Driver.getConnection().
For example:
Connection db = Driver.getConnection(url,user,pass);
// later on
Fastpath fp = ((postgresql.Connection)db).getFastpathAPI();
Class postgresql.Connection
java.lang.Object
|
+----postgresql.Connection
public class Connection extends Object implements Connection
These are the extra methods used to gain access to our extensions. I
have not listed the methods defined by java.sql.Connection.
public Fastpath getFastpathAPI() throws SQLException
This returns the Fastpath API for the current connection.
NOTE: This is not part of JDBC, but allows access to
functions on the postgresql backend itself.
It is primarily used by the LargeObject API
The best way to use this is as follows:
import postgresql.fastpath.*;
...
Fastpath fp = ((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 initialising for first time
public LargeObjectManager getLargeObjectAPI() throws SQLException
This returns the LargeObject API for the current connection.
NOTE: This is not part of JDBC, but allows access to
functions on the postgresql backend itself.
The best way to use this is as follows:
import postgresql.largeobject.*;
...
LargeObjectManager lo =
((postgresql.Connection)myconn).getLargeObjectAPI();
where myconn is an open Connection to postgresql.
Returns:
LargeObject object that implements the API
Throws: SQLException
by LargeObject when initialising 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.
NOTE: This is not part of JDBC, but an extension.
The best way to use this is as follows:
...
((postgresql.Connection)myconn).addDataType("mytype","my.class.name"-
);
...
where myconn is an open Connection to postgresql.
The handling class must extend postgresql.util.PGobject
See Also:
PGobject
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's provided
because the Large Object API uses it.
To use, you need to import the postgresql.fastpath package, using the
line:
import postgresql.fastpath.*;
Then, in your code, you need to get a FastPath object:
Fastpath fp = ((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
postgresql.Connection is required, as the getFastpathAPI() is one of
our own methods, not JDBC's.
Once you have a Fastpath instance, you can use the fastpath() methods
to execute a backend function.
Class postgresql.fastpath.Fastpath
java.lang.Object
|
+----postgresql.fastpath.Fastpath
public class Fastpath
extends Object
This class implements the Fastpath api.
This is a means of executing functions imbeded in the postgresql
backend from within a java application.
It is based around the file src/interfaces/libpq/fe-exec.c
See Also:
FastpathFastpathArg, LargeObject
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
Throws: SQLException
if a database-access error occurs.
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
prefered method to call, as function id's can/may change between
versions of the backend. For an example of how this works, refer to
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
Throws: SQLException
if name is unknown or if a database-access error
occurs.
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 lookup 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.
Parameters:
name - Function name
fnid - Function id
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.
REMEMBER to close() the resultset after calling this!!
Implementation note about function name lookups:
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 postgresql.LargeObject class performs a query upon it's
startup, and passes the returned ResultSet to the addFunctions()
method here.
Once this has been done, the LargeObject api refers to the
functions by name.
Dont think that manually converting them to the oid's will
work. Ok, 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.
Parameters:
rs - ResultSet
Throws: SQLException
if a database-access error occurs.
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.
Parameters:
name - Function name to lookup
Returns:
Function ID for fastpath call
Throws: SQLException
is function is unknown.
Class postgresql.fastpath.FastpathArg
java.lang.Object
|
+----postgresql.fastpath.FastpathArg
public class FastpathArg extends Object
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
postgresql.largeobject package
See Also:
Fastpath, LargeObjectManager, LargeObject
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.
Parameters:
s - String to store
Geometric Data Types
PostgreSQL has a set of datatypes that can store geometric features
into a table. These range from single points, lines, and polygons.
We support these types in Java with the postgresql.geometric package.
It contains classes that extend the postgresql.util.PGobject class.
Refer to that class for details on how to implement your own data type
handlers.
Class postgresql.geometric.PGbox
java.lang.Object
|
+----postgresql.util.PGobject
|
+----postgresql.geometric.PGbox
public class PGbox extends PGobject implements Serializable,
Cloneable
This represents the box datatype 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
overidden, 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 overidden 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 postgresql.geometric.PGcircle
java.lang.Object
|
+----postgresql.util.PGobject
|
+----postgresql.geometric.PGcircle
public class PGcircle extends PGobject implements Serializable,
Cloneable
This represents postgresql's circle datatype, consisting of a point
and a radius
Variables
public PGpoint center
This is the centre point
public double radius
This is the radius
Constructors
public PGcircle(double x,
double y,
double r)
Parameters:
x - coordinate of centre
y - coordinate of centre
r - radius of circle
public PGcircle(PGpoint c,
double r)
Parameters:
c - PGpoint describing the circle's centre
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 overidden 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 postgresql.geometric.PGline
java.lang.Object
|
+----postgresql.util.PGobject
|
+----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()
reuired 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 overidden 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 postgresql.geometric.PGlseg
java.lang.Object
|
+----postgresql.util.PGobject
|
+----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()
reuired 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 overidden 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 postgresql.geometric.PGpath
java.lang.Object
|
+----postgresql.util.PGobject
|
+----postgresql.geometric.PGpath
public class PGpath extends PGobject implements Serializable,
Cloneable
This implements a path (a multiple 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 overidden 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 postgresql.geometric.PGpoint
java.lang.Object
|
+----postgresql.util.PGobject
|
+----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 datatype 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 imbeded 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 overidden 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 postgresql.geometric.PGpolygon
java.lang.Object
|
+----postgresql.util.PGobject
|
+----postgresql.geometric.PGpolygon
public class PGpolygon extends PGobject implements Serializable,
Cloneable
This implements the polygon datatype 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 overidden 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
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 postgresql.largeobject package profides to Java the libpq C
interface's large object API. It consists of two classes,
LargeObjectManager, which deals with creating, opening and deleting
large obejects, and LargeObject which deals with an individual object.
Class postgresql.largeobject.LargeObject
java.lang.Object
|
+----postgresql.largeobject.LargeObject
public class LargeObject extends Object
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 postgresql.largeobject.LargeObjectManager on how to gain
access to a Large Object, or how to create one.
See Also:
LargeObjectManager
Variables
public static final int SEEK_SET
Indicates a seek from the begining 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
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.
Throws: SQLException
if a database-access error occurs.
public byte[] read(int len) throws SQLException
Reads some data from the object, and return as a byte[]
array
Parameters:
len - number of bytes to read
Returns:
byte[] array containing data read
Throws: SQLException
if a database-access error occurs.
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
Throws: SQLException
if a database-access error occurs.
public void write(byte buf[]) throws SQLException
Writes an array to the object
Parameters:
buf - array to write
Throws: SQLException
if a database-access error occurs.
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
Throws: SQLException
if a database-access error occurs.
public void seek(int pos,
int ref) throws SQLException
Sets the current position within the object.
This is similar to the fseek() call in the standard C
library.It allows you to have random access to the large object.
Parameters:
pos - position within object
ref - Either SEEK_SET, SEEK_CUR or SEEK_END
Throws: SQLException
if a database-access error occurs.
public void seek(int pos) throws SQLException
Sets the current position within the object.
This is similar to the fseek() call in the standard C
library.It allows you to have random access to the large object.
Parameters:
pos - position within object from begining
Throws: SQLException
if a database-access error occurs.
public int tell() throws SQLException
Returns:
the current position within the object
Throws: SQLException
if a database-access error occurs.
public int size() throws SQLException
This method is inefficient, as the only way to find out the
size of the object is to seek to the end, record the current position,
then return to the original position.
A better method will be found in the future.
Returns:
the size of the large object
Throws: SQLException
if a database-access error occurs.
public InputStream getInputStream() throws SQLException
Returns an InputStream from this object.
This InputStream can then be used in any method that
requires an InputStream.
Throws: SQLException
if a database-access error occurs.
public OutputStream getOutputStream() throws SQLException
Returns an OutputStream to this object
This OutputStream can then be used in any method that
requires an OutputStream.
Throws: SQLException
if a database-access error occurs.
Class postgresql.largeobject.LargeObjectManager
java.lang.Object
|
+----postgresql.largeobject.LargeObjectManager
public class LargeObjectManager extends Object
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 postgresql.largeobject.LargeObject is returned, and its
methods then allow access to the object.
This class can only be created by postgresql.Connection
To get access to this class, use the following segment of code:
import postgresql.largeobject.*;
Connection conn;
LargeObjectManager lobj;
... code that opens a connection ...
lobj = ((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 postgresql.largeobject.LargeObject on how to manipulate
the contents of a Large Object.
See Also:
LargeObject
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
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).
Parameters:
oid - of large object
Returns:
LargeObject instance providing access to the object
Throws: SQLException
on error
public LargeObject open(int oid,
int mode) throws SQLException
This opens an existing large object, based on its OID
Parameters:
oid - of large object
mode - mode of open
Returns:
LargeObject instance providing access to the object
Throws: SQLException
on error
public int create() throws SQLException
This creates a large object, returning its OID.
It defaults to READWRITE for the new object's attributes.
Returns:
oid of new object
Throws: SQLException
on error
public int create(int mode) throws SQLException
This creates a large object, returning its OID
Parameters:
mode - a bitmask describing different attributes of
the
new object
Returns:
oid of new object
Throws: SQLException
on error
public void delete(int oid) throws SQLException
This deletes a large object.
Parameters:
oid - describing object to delete
Throws: SQLException
on error
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.
Parameters:
oid - describing object to delete
Throws: SQLException
on error
Object Serialisation
PostgreSQL is not a normal SQL Database. It is far 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)
Ok, 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 LargeObject to store them.
However, you cannot perform queries on those objects.
What the 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 postgresql.util.Serialize
java.lang.Object
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+----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
ordeserialize 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 postgresql.util package contains classes used by the internals of
the main driver, and the other extensions.
Class postgresql.util.PGmoney
java.lang.Object
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+----postgresql.util.PGobject
|
+----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 overidden 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 postgresql.util.PGobject
java.lang.Object
|
+----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 postgresql.Connection permits a class that extends this
class to be associated with a named type. This is how the
postgresql.geometric package operates.
ResultSet.getObject() will return this class for any type that is
not recognised on having it's own handler. Because of this, any
postgresql data type is supported.
Constructors
public PGobject()
This is called by 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
overidden.
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 overidden, 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 overidden 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 overidden to allow the object to be cloned
Overrides:
clone in class Object
public String toString()
This is defined here, so user code need not overide it.
Returns:
the value of this object, in the syntax expected by
postgresql
Overrides:
toString in class Object
Class postgresql.util.PGtokenizer
java.lang.Object
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+----postgresql.util.PGtokenizer
public class PGtokenizer extends Object
This class is used to tokenize the text output of postgres.
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 tokeniser.
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 datatypes 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 postgresql.util.Serialize
This was documented earlier under Object Serialisation.
Class postgresql.util.UnixCrypt
java.lang.Object
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+----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 cleartext 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 randomised.
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 cleartext 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 aquired).
Returns:
true if the password should be considered correct.
Using the driver in a multi Threaded or Servlet environment
A problem with many JDBC drivers, is that only one thread can use a
Connection at any one time - otherwise a thread could send a query
while another one is receiving results, and this would be a bad thing
for the database engine.
PostgreSQL 6.4, brings thread safety to the entire driver. Standard
JDBC was thread safe in 6.3.x, but the Fastpath
API wasn't.
So, if your application uses multiple threads (which most decent ones
would), then you don't have to worry about complex schemes to ensure
only one uses the database at any time.
If a thread attempts to use the connection while another is using it,
it will wait until the other thread has finished it's current
operation.
If it's a standard SQL statement, then the operation is sending the
statement, and retrieving any ResultSet (in full).
If it's a Fastpath call (ie: reading a block from a LargeObject), then
it's the time to send, and retrieve that block.
This is fine for applications & applets, but can cause a performance
problem with servlets.
With servlets, you can have a heavy load on the connection. If you
have several threads performing queries, then each one will pause,
which may not be what you are after.
To solve this, you would be advised to create a pool of Connections.
When ever a thread needs to use the database, it asks a manager class
for a Connection. It hands a free connection to the thread, and marks
it as busy. If a free connection is not available, it opens one.
Once the thread has finished with it, it returns it to the manager,
who can then either close it, or add it to the pool. The manager would
also check that the connection is still alive, and remove it from the
pool if it's dead.
So, with servlets, it's up to you to use either a single connection,
or a pool. The plus side for a pool is that threads will not be hit by
the bottle neck caused by a single network connection. The down side,
is that it increases the load on the server, as a backend is created
for each Connection.
It's up to you, and your applications requirements. |