Oracle8i JDBC Developer's Guide and Reference
Release 3 (8.1.7)
Part Number A83724-01

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Working with BLOBs and CLOBs

This section describes how to read and write data to and from binary large objects (BLOBs) and character large objects (CLOBs) in an Oracle database, using LOB locators.

For general information about Oracle8i LOBs and how to use them, see the Oracle8i Application Developer's Guide--Large Objects (LOBs).

Getting and Passing BLOB and CLOB Locators

Standard as well as Oracle-specific getter and setter methods are available for retrieving or passing LOB locators from or to the database.

Retrieving BLOB and CLOB Locators

Given a standard JDBC result set (java.sql.ResultSet) or callable statement (java.sql.CallableStatement) that includes BLOB or CLOB locators, you can access the locators by using standard getter methods, as follows. All the standard and Oracle-specific getter methods discussed here take either an int column index or a String column name as input.

If you retrieve or cast the result set or callable statement to an OracleResultSet or OracleCallableStatement object, then you can use Oracle extensions as follows:

Example: Getting BLOB and CLOB Locators from a Result Set

Assume the database has a table called lob_table with a column for a BLOB locator, blob_col, and a column for a CLOB locator, clob_col. This example assumes that you have already created the Statement object, stmt.

First, select the LOB locators into a standard result set, then get the LOB data into appropriate Java classes: 

// Select LOB locator into standard result set.
ResultSet rs =
   stmt.executeQuery ("SELECT blob_col, clob_col FROM lob_table");
while (rs.next())
{
   // Get LOB locators into Java wrapper classes.
   java.sql.Blob blob = (java.sql.Blob)rs.getObject(1);
   java.sql.Clob clob = (java.sql.Clob)rs.getObject(2);
   (...process...)
}

The output is cast to java.sql.Blob and Clob. As an alternative, you can cast the output to oracle.sql.BLOB and CLOB to take advantage of extended functionality offered by the oracle.sql.* classes. For example, you can rewrite the above code to get the LOB locators as: 

   // Get LOB locators into Java wrapper classes.
   oracle.sql.BLOB blob = (BLOB)rs.getObject(1);
   oracle.sql.CLOB clob = (CLOB)rs.getObject(2);
   (...process...)
Example: Getting a CLOB Locator from a Callable Statement

The callable statement methods for retrieving LOBs are identical to the result set methods.

For example, if you have an OracleCallableStatement ocs that calls a function func that has a CLOB output parameter, then set up the callable statement as in the following example.

This example registers OracleTypes.CLOB as the typecode of the output parameter. 

OracleCallableStatement ocs = 
   (OracleCallableStatement)conn.prepareCall("{? = call func()}");
ocs.registerOutParameter(1, OracleTypes.CLOB);
ocs.execute();
oracle.sql.CLOB clob = ocs.getCLOB(1);

Passing BLOB and CLOB Locators

Given a standard JDBC prepared statement (java.sql.PreparedStatement) or callable statement (java.sql.CallableStatement), you can use standard setter methods to pass LOB locators, as follows. All the standard and Oracle-specific setter methods discussed here take an int parameter index and the LOB locator as input.

Given an Oracle-specific OraclePreparedStatement or OracleCallableStatement, then you can use Oracle extensions as follows:

Example: Passing a BLOB Locator to a Prepared Statement

If you have an OraclePreparedStatement object ops and a BLOB named my_blob, then write the BLOB to the database as follows: 

OraclePreparedStatement ops = (OraclePreparedStatement)conn.prepareStatement
                            ("INSERT INTO blob_table VALUES(?)"); 
ops.setBLOB(1, my_blob);
ops.execute();
Example: Passing a CLOB Locator to a Callable Statement

If you have an OracleCallableStatement object ocs and a CLOB named my_clob, then input the CLOB to the stored procedure proc as follows: 

OracleCallableStatement ocs = 
   (OracleCallableStatement)conn.prepareCall("{call proc(?))}");
ocs.setClob(1, my_clob);
ocs.execute();

Reading and Writing BLOB and CLOB Data

Once you have a LOB locator, you can use JDBC methods to read and write the LOB data. LOB data is materialized as a Java array or stream. However, unlike most Java streams, a locator representing the LOB data is stored in the table. Thus, you can access the LOB data at any time during the life of the connection.

To read and write the LOB data, use the methods in the oracle.sql.BLOB or oracle.sql.CLOB class, as appropriate. These classes provide functionality such as reading from the LOB into an input stream, writing from an output stream into a LOB, determining the length of a LOB, and closing a LOB.

Notes:

  • To write LOB data, the application must acquire a write lock on the LOB object. One way to accomplish this is through a SELECT FOR UPDATE. Also, disable auto-commit mode.

  • The implementation of the data access API uses direct native calls in the JDBC OCI and server-side internal drivers, thereby providing better performance. You can use the same API on the LOB classes in all Oracle JDBC drivers.

  • In the case of the JDBC Thin driver only, the implementation of the data access API uses the PL/SQL DBMS_LOB package internally. You never have to use DBMS_LOB directly. This is in contrast to the 8.0.x drivers. For more information on the DBMS_LOB package, see the Oracle8i Supplied PL/SQL Packages Reference.

 

To read and write LOB data, you can use these methods:

Example: Reading BLOB Data

Use the getBinaryStream() method of the oracle.sql.BLOB class to read BLOB data. The getBinaryStream() method reads the BLOB data into a binary stream.

The following example uses the getBinaryStream() method to read BLOB data into a byte stream and then reads the byte stream into a byte array (returning the number of bytes read, as well). 

// Read BLOB data from BLOB locator.
InputStream byte_stream = my_blob.getBinaryStream();
byte [] byte_array = new byte [10];
int bytes_read = byte_stream.read(byte_array);
...
Example: Reading CLOB Data

The following example uses the getCharacterStream() method to read CLOB data into a Unicode character stream. It then reads the character stream into a character array (returning the number of characters read, as well). 

// Read CLOB data from CLOB locator into Reader char stream.
Reader char_stream = my_clob.getCharacterStream();
char [] char_array = new char [10];
int chars_read = char_stream.read (char_array, 0, 10);
...

The next example uses the getAsciiStream() method of the oracle.sql.CLOB class to read CLOB data into an ASCII character stream. It then reads the ASCII stream into a byte array (returning the number of bytes read, as well). 

// Read CLOB data from CLOB locator into Input ASCII character stream
Inputstream asciiChar_stream = my_clob.getAsciiStream(); 
byte[] asciiChar_array = new byte[10]; 
int asciiChar_read = asciiChar_stream.read(asciiChar_array,0,10);
Example: Writing BLOB Data

Use the getBinaryOutputStream() method of an oracle.sql.BLOB object to write BLOB data.

The following example reads a vector of data into a byte array, then uses the getBinaryOutputStream() method to write an array of character data to a BLOB. 

java.io.OutputStream outstream;

// read data into a byte array 
byte[] data = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

// write the array of binary data to a BLOB
outstream = ((BLOB)my_blob).getBinaryOutputStream();
outstream.write(data);
...
Example: Writing CLOB Data

Use the getCharacterOutputStream() method or the getAsciiOutputStream() method to write data to a CLOB. The getCharacterOutputStream() method returns a Unicode output stream; the getAsciiOutputStream() method returns an ASCII output stream.

The following example reads a vector of data into a character array, then uses the getCharacterOutputStream() method to write the array of character data to a CLOB. The getCharacterOutputStream() method returns a java.io.Writer instance in an oracle.sql.CLOB object, not a java.sql.Clob object. 

java.io.Writer writer;

// read data into a character array
char[] data = {'0','1','2','3','4','5','6','7','8','9'};

// write the array of character data to a CLOB 
writer = ((CLOB)my_clob).getCharacterOutputStream();
writer.write(data);
writer.flush();
writer.close();
...

The next example reads a vector of data into a byte array, then uses the getAsciiOutputStream() method to write the array of ASCII data to a CLOB. Because getAsciiOutputStream() returns an ASCII output stream, you must cast the output to a oracle.sql.CLOB datatype. 

java.io.OutputStream out;

// read data into a byte array
byte[] data = {'0','1','2','3','4','5','6','7','8','9'};

// write the array of ascii data to a CLOB 
out = ((CLOB)clob).getAsciiOutputStream();
out.write(data);
out.flush();
out.close();

Creating and Populating a BLOB or CLOB Column

Create and populate a BLOB or CLOB column in a table by using SQL statements.

Note:

You cannot create a new BLOB or CLOB locator in your application, such as with a Java new statement. You must create the locator through a SQL operation, and then select it into your application.  

Create a BLOB or CLOB column in a table with the SQL CREATE TABLE statement, then populate the LOB. This includes creating the LOB entry in the table, obtaining the LOB locator, creating a file handler for the data (if you are reading the data from a file), and then copying the data into the LOB.

Creating a BLOB or CLOB Column in a New Table

To create a BLOB or CLOB column in a new table, execute the SQL CREATE TABLE statement. The following example code creates a BLOB column in a new table. This example assumes that you have already created your Connection object conn and Statement object stmt: 

String cmd = "CREATE TABLE my_blob_table (x varchar2 (30), c blob)";
stmt.execute (cmd);

In this example, the VARCHAR2 column designates a row number, such as 1 or 2, and the BLOB column stores the locator of the BLOB data.

Populating a BLOB or CLOB Column in a New Table

This example demonstrates how to populate a BLOB or CLOB column by reading data from a stream. These steps assume that you have already created your Connection object conn and Statement object stmt. The table my_blob_table is the table that was created in the previous section.

The following example writes the GIF file john.gif to a BLOB.

  1. Begin by using SQL statements to create the BLOB entry in the table. Use the empty_blob syntax to create the BLOB locator. 

    stmt.execute ("INSERT INTO my_blob_table VALUES ('row1', empty_blob())");
  2. Get the BLOB locator from the table. 

    BLOB blob;
cmd = "SELECT * FROM my_blob_table WHERE X='row1'";
ResultSet rest = stmt.executeQuery(cmd);
BLOB blob = ((OracleResultSet)rset).getBLOB(2);
  3. Declare a file handler for the john.gif file, then print the length of the file. This value will be used later to ensure that the entire file is read into the BLOB. Next, create a FileInputStream object to read the contents of the GIF file, and an OutputStream object to retrieve the BLOB as a stream. 

    File binaryFile = new File("john.gif");
System.out.println("john.gif length = " + binaryFile.length());
FileInputStream instream = new FileInputStream(binaryFile);
OutputStream outstream = blob.getBinaryOutputStream();
  4. Call getBufferSize() to retrieve the ideal buffer size (according to calculations by the JDBC driver) to use in writing to the BLOB, then create the buffer byte array. 

    int size = blob.getBufferSize();
byte[] buffer = new byte[size];
int length = -1;
  5. Use the read() method to read the GIF file to the byte array buffer, then use the write() method to write it to the BLOB. When you finish, close the input and output streams. 

    while ((length = instream.read(buffer)) != -1)
   outstream.write(buffer, 0, length);
instream.close();
outstream.close();

Once your data is in the BLOB or CLOB, you can manipulate the data. This is described in the next section, "Accessing and Manipulating BLOB and CLOB Data".

Accessing and Manipulating BLOB and CLOB Data

Once you have your BLOB or CLOB locator in a table, you can access and manipulate the data to which it points. To access and manipulate the data, you first must select their locators from a result set or from a callable statement. "Getting and Passing BLOB and CLOB Locators" describes these techniques in detail.

After you select the locators, you can retrieve the BLOB or CLOB data. You will usually want to cast the result set to the OracleResultSet datatype so that you can retrieve the data in oracle.sql.* format. After retrieving the BLOB or CLOB data, you can manipulate it however you want.

This example is a continuation of the example in the previous section. It uses the SQL SELECT statement to select the BLOB locator from the table my_blob_table into a result set. The result of the data manipulation is to print the length of the BLOB in bytes. 

// Select the blob - what we are really doing here
// is getting the blob locator into a result set 
BLOB blob;
cmd = "SELECT *  FROM my_blob_table";
ResultSet rset = stmt.executeQuery (cmd);

// Get the blob data - cast to OracleResult set to 
// retrieve the data in oracle.sql format 
String index = ((OracleResultSet)rset).getString(1);
blob = ((OracleResultSet)rset).getBLOB(2);

// get the length of the blob
int length = blob.length();

// print the length of the blob
System.out.println("blob length" + length);

// read the blob into a byte array 
// then print the blob from the array
byte bytes[] = blob.getBytes(1, length);
printBytes(bytes, length);

Additional BLOB and CLOB Features

In addition to what has already been discussed in this chapter, the oracle.sql.BLOB and CLOB classes have a number of methods for further functionality.

Note:

The oracle.sql.CLOB class supports all the character sets that the Oracle data server supports for CLOB types.  

Additional BLOB Methods

The oracle.sql.BLOB class includes the following methods:

Additional CLOB Methods

The oracle.sql.CLOB class includes the following methods:

Creating Empty LOBs

Before writing data to an internal LOB, you must make sure the LOB column/attribute is not null: it must contain a locator. You can accomplish this by initializing the internal LOB as an empty LOB in an INSERT or UPDATE statement, using the empty_lob() method defined in the oracle.sql.BLOB and oracle.sql.CLOB classes:

A JDBC driver creates an empty LOB instance without making database round trips. You can use empty LOBs in the following:

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