Support for Streams

Standard SQLJ provides three specialized classes, listed below, for convenient handling of long data in streams. These stream types can be used for iterator columns to retrieve data from the database, or for input host variables to send data to the database. As with Java streams in general, these classes allow the convenience of handling and transferring large data items in manageable chunks.

• BinaryStream

• AsciiStream

• UnicodeStream

These classes are in the sqlj.runtime package.

This section discusses general use of these classes, Oracle SQLJ extended functionality, and stream class methods.

General Use of SQLJ Streams

With respect to an Oracle8i database, Table 5-1 in "Supported Types for Host Expressions" lists the datatypes you would typically process using these stream classes. To summarize: AsciiStream and UnicodeStream are typically used for datatype LONG (java.sql.Types.LONGVARCHAR) but might also be used for datatype VARCHAR2 (Types.VARCHAR); BinaryStream is typically used for datatype LONG RAW (Types.LONGVARBINARY) but might also be used for datatype RAW (Types.BINARY or Types.VARBINARY).

Of course, any use of streams is at your discretion. As Table 5-1 documents, LONG and VARCHAR2 data can also be manifested in Java strings, while RAW and LONGRAW data can also be manifested in Java byte arrays. Furthermore, if your database supports large object types such as BLOB (binary large object) and CLOB (character large object), you may find these to be preferable to using types such as LONG and LONG RAW (although streams may still be used in extracting data from large objects). Oracle8i supports large object types--see "Support for BLOB, CLOB, and BFILE".

All three SQLJ stream classes are subclasses of the standard Java input stream class, java.io.InputStream, and act as wrappers to provide the functionality required by SQLJ. This functionality is to communicate to SQLJ the type and length of data in the underlying stream so that it can be handled and formatted properly.

You can use the SQLJ stream types for host variables to send data to the database or iterator columns to receive data from the database.

Using SQLJ Streams to Send Data to the Database

Standard SQLJ allows you to use streams as host variables to update the database.

A key point in sending a SQLJ stream to the database is that you must somehow determine the length of the data and specify that length to the constructor of the SQLJ stream. This will be further discussed below.

You can use a SQLJ stream to send data to the database as follows:

1. Determine the length of your data.

2. Create a standard Java input stream--an instance ofjava.io.InputStream or some subclass--as you normally would.

3. Create an instance of the appropriate SQLJ stream class (depending on the type of data), passing the input stream and length (as an int) to the constructor.

4. Use the SQLJ stream instance as a host variable in a suitable SQL operation in a SQLJ executable statement.

5. Close the stream (this is not required but is recommended).

This section now goes into more detail regarding two typical examples of sending a SQLJ stream to the database:

• Using an operating system file to update a LONG or LONG RAW column. (This can be either a binary file to update a LONG RAW column or an ASCII or Unicode file to update a LONG column.)

• Using a byte array to update a LONG RAW column.

Updating LONG or LONG RAW from a File

In updating a database column (presumably a LONG or LONG RAW column) from a file, a step is needed to determine the length. You can do this by creating a java.io.File object before you create your input stream.

Here are the steps in updating the database from a file:

1. Create a java.io.File object from your file. You can specify the file path name to the File class constructor.

2. Use the length() method of the File object to determine the length of the data. This method returns a long value, which you must cast to an int for input to the SQLJ stream class constructor.

3. Create a java.io.FileInputStream object from your File object. You can pass the File object to the FileInputStream constructor.

4. Create an appropriate SQLJ stream object. This would be a BinaryStream object for a binary file, an AsciiStream object for an ASCII file, or a UnicodeStream object for a Unicode file. Pass the FileInputStream object and data length (as an int) to the SQLJ stream class constructor.

   The SQLJ stream constructor signatures are all identical, as follows:

   BinaryStream (InputStream in, int length)
   AsciiStream (InputStream in, int length)
   UnicodeStream (InputStream in, int length)
   
   

   An instance of java.io.InputStream or of any subclass, such as FileInputStream, can be input to these constructors.

5. Use the SQLJ stream object as a host variable in an appropriate SQL operation in a SQLJ executable statement.

The following is an example of writing LONG data to the database from a file. Presume you have an HTML file in /private/mydir/myfile.html and you want to insert the file contents into a LONG column called asciidata in a database table named filetable.

Imports:

import java.io.*;
import sqlj.runtime.*;

Executable code:

File myfile = new File ("/private/mydir/myfile.html");
int length = (int)myfile.length();     // Must cast long output to int.
FileInputStream fileinstream = new FileInputStream(myfile);
AsciiStream asciistream = new AsciiStream(fileinstream, length);
#sql { INSERT INTO filetable (asciidata) VALUES (:asciistream) };
asciistream.close();
...

Updating LONG RAW from a Byte Array

You must determine the length of the data before updating the database from a byte array. (Presumably you would be updating a LONG RAW column.) This is more trivial for arrays than for files, though, because all Java arrays have functionality to return the length.

Here are the steps in updating the database from a byte array:

1. Use the length functionality of the array to determine the length of the data. This returns an int, which is what you will need for the constructor of any of the SQLJ stream classes.

2. Create a java.io.ByteArrayInputStream object from your array. You can pass the byte array to the ByteArrayInputStream constructor.

3. Create a BinaryStream object. Pass the ByteArrayInputStream object and data length (as an int) to the BinaryStream class constructor.

   The constructor signature is as follows:

   BinaryStream (InputStream in, int length)
   
   

   You can use an instance of java.io.InputStream or of any subclass, such as ByteArrayInputStream.

4. Use the SQLJ stream object as a host variable in an appropriate SQL operation in a SQLJ executable statement.

The following is an example of writing LONG RAW data to the database from a byte array. Presume you have a byte array bytearray[] and you want to insert its contents into a LONG RAW column called bindata in a database table named bintable.

Imports:

import java.io.*;
import sqlj.runtime.*;

Executable code:

byte[] bytearray = new byte[100];

(Populate bytearray somehow.)
...
int length = bytearray.length;
ByteArrayInputStream arraystream = new ByteArrayInputStream(bytearray);
BinaryStream binstream = new BinaryStream(arraystream, length);
#sql { INSERT INTO bintable (bindata) VALUES (:binstream) };
binstream.close();
...

Retrieving Data into Streams--Precautions

You can also use the SQLJ stream classes to retrieve data from the database, but the logistics of using streams make certain precautions necessary with some database products.

When reading long data and writing it to a stream using an Oracle8i database and Oracle JDBC driver, you must be careful in how you access and process the stream data.

As the Oracle JDBC drivers access data from an iterator row, they must flush any stream item from the communications pipe before accessing the next data item. Even though the stream data is written to a local stream as the iterator row is processed, this stream data will be lost if you do not read it from the local stream before the JDBC driver accesses the next data item. This is because of the way streams must be processed due to their potentially large size and unknown length.

Therefore, as soon as your Oracle JDBC driver has accessed a stream item and written it to a local stream variable, you must read and process the local stream before anything else is accessed from the iterator.

This is especially problematic in using positional iterators, with their requisite FETCH INTO syntax. With each fetch, all columns are read before any are processed. Therefore, there can be only one stream item and it must be the last item accessed.

To summarize the precautions you must take:

• When using a positional iterator, you can only have one stream column and it must be the last column. As soon as you have fetched each row of the iterator, writing the stream item to a local input stream variable in the process, you must read and process the local stream variable before advancing to the next row of the iterator.

• When using a named iterator, you can have multiple stream columns; however, as you process each iterator row, each time you access a stream field, writing the data to a local stream variable in the process, you must read and process the local stream immediately, before reading anything else from the iterator.

  Furthermore, in processing each row of a named iterator, you must call the column accessor methods in the same order in which the database columns were selected in the query that populated the iterator. As mentioned in similar preceding discussion, this is because stream data remains in the communications pipe after the query. If you try to access columns out of order, then the stream data may be skipped over and lost in the course of accessing other columns.

Using SQLJ Streams to Retrieve Data from the Database

To retrieve data from a database column as a stream, standard SQLJ allows you to select data into a named or positional iterator that has a column of the appropriate SQLJ stream type.

This section covers the basic steps in retrieving data into a SQLJ stream using a positional iterator or a named iterator. This discussion takes into account the precautions documented in "Retrieving Data into Streams--Precautions".

These are general steps. For more information, see "Processing SQLJ Streams" and "Examples of Retrieving and Processing Stream Data".

Using a SQLJ Stream Column in a Positional Iterator

Use the following steps to retrieve data into a SQLJ stream using a positional iterator:

1. Declare a positional iterator class with the last column being of the appropriate SQLJ stream type.

2. Declare a local variable of your iterator type.

3. Declare a local variable of the appropriate SQLJ stream type. This will be used as a host variable to receive data from each row of the SQLJ stream column of the iterator.

4. Query the database to populate the iterator you declared in step 2.

5. Process the iterator as usual (see "Using Positional Iterators"). Because the host variables in the INTO-list of the FETCH INTO statement must be in the same order as the columns of the positional iterator, the local input stream variable is the last host variable in the list.

6. In the iterator processing loop, after each iterator row is accessed, immediately read and process the local input stream, storing or outputting the stream data as desired.

7. Close the local input stream each time through the iterator processing loop (this is not required but is recommended).

8. Close the iterator.

Using SQLJ Stream Columns in a Named Iterator

Use the following steps to retrieve data into one or more SQLJ streams using a named iterator:

1. Declare a named iterator class with one or more columns of appropriate SQLJ stream type.

2. Declare a local variable of your iterator type.

3. Declare a local variable of some input stream type for each SQLJ stream column in the iterator. These will be used to receive data from the stream-column accessor methods. These local stream variables do not have to be SQLJ stream types; they can be standard java.io.InputStream if desired. (They do not have to be SQLJ stream types because the issue of correctly formatting the data from the database was already taken care of as a result of the iterator columns being of appropriate SQLJ stream types.)

4. Query the database to populate the iterator you declared in step 2.

5. Process the iterator as usual (see "Using Named Iterators"). In processing each row of the iterator, as each stream-column accessor method returns the stream data, write it to the corresponding local input stream variable you declared in step 3.

   To ensure that stream data will not be lost, call the column accessor methods in the same order in which columns were selected in the query in step 4.

6. In the iterator processing loop, immediately after calling the accessor method for any stream column and writing the data to a local input stream variable, read and process the local input stream, storing or outputting the stream data as desired.

7. Close the local input stream each time through the iterator processing loop (this is not required but is recommended).

8. Close the iterator.

Processing SQLJ Streams

In processing a SQLJ stream column in a named or positional iterator, the local stream variable used to receive the stream data can be either a SQLJ stream type or the standard java.io.InputStream type. In either case, standard input stream methods are supported.

If the local stream variable is a SQLJ stream type--BinaryStream, AsciiStream, or UnicodeStream--you have the option of either reading data directly from the SQLJ stream object, or retrieving the underlying java.io.InputStream object and reading data from that. This is just a matter of preference--the former approach is simpler; the latter approach involves more direct and efficient data access.

The following important methods of the InputStream class--the skip() method, close() method, and three forms of the read() method--are supported by the SQLJ stream classes as well.

• int read ()--Reads the next byte of data from the input stream. The byte of data is returned as an int value in the range 0 to 255. If the end of the stream has already been reached, the value -1 is returned. This method blocks until one of the following: 1) input data is available; 2) the end of the stream is detected; or 3) an exception is thrown.

• int read (byte b[])--Reads up to b.length bytes of data from the input stream, writing the data into the specified b[] byte array. It returns an int value indicating how many bytes were read or -1 if the end of the stream has already been reached. This method blocks until input is available.

• int read (byte b[], int off, int len)--Reads up to len (length) bytes of data from the input stream, starting at the byte specified by the offset, off, and writing the data into the specified b[] byte array. It returns an int value indicating how many bytes were read or -1 if the end of the stream has already been reached. This method blocks until input is available.

• long skip (long n)--Skips over and discards n bytes of data from the input stream. In some circumstances, however, this method will actually skip a smaller number of bytes. It returns a long value indicating the actual number of bytes skipped.

• void close()--Closes the stream and releases any associated resources.

In addition, SQLJ stream classes support the following important method:

• InputStream getInputStream()--Returns the underlying input stream being wrapped, as a java.io.InputStream object.

Examples of Retrieving and Processing Stream Data

This section provides examples of various scenarios of retrieving stream data from the database, as follows:

• using a SELECT statement to select data from a LONG column and populate a SQLJ AsciiStream column in a named iterator

• using a SELECT statement to select data from a LONG RAW column and populate a SQLJ BinaryStream column in a positional iterator

Example: Selecting LONG Data into AsciiStream Column of Named Iterator

This example selects data from a LONG database column, populating a SQLJ AsciiStream column in a named iterator.

Presume there is a table named filetable with a VARCHAR2 column called filename that contains file names, and a LONG column called filecontents that contains file contents in ASCII.

Imports and declarations:

import sqlj.runtime.*;
import java.io.*;

#sql iterator MyNamedIter (String filename, AsciiStream filecontents);

Executable code:

MyNamedIter namediter = null;
String fname;
AsciiStream ascstream;
#sql namediter = { SELECT filename, filecontents FROM filetable };
while (namediter.next()) {
   fname = namediter.filename();
   ascstream = namediter.filecontents();
   System.out.println("Contents for file " + fname + ":");
   printStream(ascstream);
   ascstream.close();
}
namediter.close();
...
public void printStream(InputStream in) throws IOException
{
   int asciichar;
   while ((asciichar = in.read()) != -1) {
      System.out.print((char)asciichar);
   }
}

Remember that you can pass a SQLJ stream to any method that takes a standard java.io.InputStream as an input parameter.

Example: Selecting LONG RAW Data into BinaryStream Column of Positional Iterator

This example selects data from a LONG RAW database column, populating a SQLJ BinaryStream column in a positional iterator.

As explained in "Retrieving Data into Streams--Precautions", there can be only one stream column in a positional iterator and it must be the last column.

Presume there is a table named bintable with a NUMBER column called identifier and a LONG RAW column called bindata that contains binary data associated with the identifier.

Imports and declarations:

import sqlj.runtime.*;

#sql iterator MyPosIter (int, BinaryStream);

Executable code:

MyPosIter positer = null;
int id=0;
BinaryStream binstream=null;
#sql positer = { SELECT identifier, bindata FROM bintable };
while (true) {
   #sql { FETCH :positer INTO :id, :binstream };
   if (positer.endFetch()) break;
   
   (...process data as desired...)
   
   binstream.close();
}
positer.close();
...

SQLJ Stream Objects as Output Parameters and Function Return Values

As described in the preceding sections, standard SQLJ supports use of the BinaryStream, AsciiStream, and UnicodeStream classes in the package sqlj.runtime for retrieval of stream data into iterator columns.

In addition, the Oracle SQLJ implementation allows the following uses of SQLJ stream types if you are using an Oracle database, Oracle JDBC driver, and the Oracle customizer:

• They can appear as OUT or INOUT host variables from a stored procedure or function call.

• They can appear as the return value from a stored function call.

Streams as Stored Procedure Output Parameters

You can use the types AsciiStream, BinaryStream and UnicodeStream as the assignment type for a stored procedure or stored function OUT or INOUT parameter.

Presume the following table definition:

CREATE TABLE streamexample (name VARCHAR2 (256), data LONG);
INSERT INTO streamexample (data, name)
   VALUES
   ('0000000000111111111112222222222333333333344444444445555555555',
   'StreamExample');

Also presume the following stored procedure definition, which uses the streamexample table:

CREATE OR REPLACE PROCEDURE out_longdata 
                            (dataname VARCHAR2, longdata OUT LONG) IS
BEGIN
   SELECT data INTO longdata FROM streamexample WHERE name = dataname;
END out_longdata;

The following sample code uses a call to the out_longdata stored procedure to read the long data.

Imports:

import sqlj.runtime.*;

Executable code:

AsciiStream data;
#sql { CALL out_longdata('StreamExample', :OUT data) };
int c;
while ((c = data.read ()) != -1)
   System.out.print((char)c);
System.out.flush();
data.close();
...

Streams as Stored Function Results

You can use the types AsciiStream, BinaryStream and UnicodeStream as the assignment type for a stored function return result.

Presume the same streamexample table definition as in the preceding stored procedure example.

Also presume the following stored function definition which uses the streamexample table:

CREATE OR REPLACE FUNCTION get_longdata (dataname VARCHAR2) RETURN long
   IS longdata LONG;
BEGIN
   SELECT data INTO longdata FROM streamexample WHERE name = dataname;
   RETURN longdata;
END get_longdata;

The following sample code uses a call to the get_longdata stored function to read the long data.

Imports:

import sqlj.runtime.*;

Executable code:

AsciiStream data;
#sql data = { VALUES(get_longdata('StreamExample')) };
int c;
while ((c = data.read ()) != -1)
   System.out.print((char)c);
System.out.flush();
data.close();
...

Stream Class Methods

The SQLJ stream classes in the sqlj.runtime package--BinaryStream, AsciiStream, and UnicodeStream--are all subclasses of the sqlj.runtime.StreamWrapper class.

The StreamWrapper class provides the following methods that are inherited by the SQLJ stream classes:

• InputStream getInputStream()--As discussed in "Processing SQLJ Streams", you can optionally use this method to get the underlying java.io.InputStream object of any SQLJ stream object. This is not required, however, as you can also process SQLJ stream objects directly.

• void setLength(int length)--You can use this to set the length attribute of a SQLJ stream object. This is not necessary if you have already set length in constructing the stream object, unless you want to change it for some reason.

  Bear in mind that the length attribute must be set to an appropriate value before you send a SQLJ stream to the database.

• int getLength()--This method returns the value of the length attribute of a SQLJ stream. This value is only meaningful if you explicitly set it using the stream object constructor or the setLength() method. When you retrieve data into a stream, the length attribute is not set automatically.

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