| Oracle® Database JDBC Developer's Guide 11g Release 2 (11.2) E16548-03 |
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PDF · Mobi · ePub |
| Oracle® Database JDBC Developer's Guide 11g Release 2 (11.2) E16548-03 |
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PDF · Mobi · ePub |
This chapter describes how to use Java Database Connectivity (JDBC) to access and manipulate large objects (LOB) using either the data interface or the locator interface.
In previous releases, Oracle JDBC drivers required Oracle extensions to standard JDBC types to perform many operations in the Oracle Database. JDBC 3.0 reduced the requirement of using Oracle extensions and JDBC 4.0 nearly eliminated this limitation. Refer to the Javasoft Javadoc for the java.sql and javax.sql packages, and to the Oracle JDBC Javadoc for details on Oracle extensions.
This chapter contains the following sections:
Prior to Oracle Database 10g, the maximum size of a LOB was 2^32 bytes. This restriction has been removed since Oracle Database 10g, and the maximum size is limited to the size of available physical storage.
See Also:
Oracle Database SecureFiles and Large Objects Developer's GuideThe Oracle database supports the following four LOB data types:
Binary large object (BLOB)
This data type is used for unstructured binary data.
Character large object (CLOB)
This data type is used for character data.
National character large object (NCLOB)
This data type is used for national character data.
BFILE
This data type is used for large binary data objects stored in operating system files, outside of database tablespaces.
BLOBs, CLOBs, and NCLOBs are stored persistently in a database tablespace and all operations performed on these data types are under transaction control.
BFILE is an Oracle proprietary data type that provides read-only access to data located outside the database tablespaces on tertiary storage devices, such as hard disks, network mounted files systems, CD-ROMs, PhotoCDs, and DVDs. BFILE data is not under transaction control and is not stored by database backups.
The PL/SQL language supports the LOB data types and the JDBC interface allows passing IN parameters to PL/SQL procedures or functions, and retrieval of OUT parameters or returns. PL/SQL uses value semantics for all data types including LOBs, but reference semantics only for BFILE.
Oracle Database 11g Release 1 (11.1) introduced Oracle SecureFiles, a completely new storage for LOBs.
Following Features of Oracle SecureFiles are transparently available to JDBC programs through the existing APIs:
SecureFile compression enables users to compress data to save disk space.
SecureFile encryption introduces a new encryption facility that allows for random reads and writes of the encrypted data.
Deduplication enables Oracle database to automatically detect duplicate LOB data and conserve space by storing only one copy of data.
LOB data path optimization includes logical cache above storage layer and new caching modes.
High performance space management.
The setLobOptions and getLobOptions APIs are described in the PL/SQL Packages and Types Reference, and may be accessed from JDBC through callable statements.
Following Features of Oracle SecureFiles are implemented in the database through updation to the existing APIs:
Starting from Oracle Database 11g Release 2 (11.2), you can check whether or not your BLOB or CLOB data uses Oracle SecureFile storage. To achieve this, use the following method from oracle.sql.BLOB or oracle.sql.CLOB class:
public boolean isSecureFile() throws SQLException
If this method returns true, then your data uses SecureFile storage.
Zero-Copy I/O for Oracle SecureFiles
With the release of Oracle Database 11g Release 2 (11.2) JDBC Drivers, the performance of Oracle SecureFiles operations is greatly improved because Oracle Net Services now uses zero-copy I/O framework for better buffer management.
Oracle Database 11g Release 2 (11.2) introduces a new connection property oracle.net.useZeroCopyIO. This property can be used to enable or disable the zero-copy I/O protocol. This connection property is defined as the following constant: OracleConnection.CONNECTION_PROPERTY_THIN_NET_USE_ZERO_COPY_IO. If you want to disable the zero-copy I/O framework, then set the value of this connection property to false. By default, the value of this connection property is true.
This section describes the following topics:
The Oracle 11.2 drivers provide a streamlined mechanism for writing and reading the entire LOB contents. This is referred to as the data interface. The data interface uses standard JDBC methods such as getString and setBytes to read and write LOB data. It is simpler to code and faster in many cases. Unlike the standard java.sql.Blob, java.sql.Clob and java.sql.NClob interfaces, it does not provide random access capability and cannot access data beyond 2147483648 elements.
In Oracle Database 11g release 2 (11.2), the setBytes, setBinaryStream, setString, setCharacterStream, and setAsciiStream methods of PreparedStatement are extended to enhance the ability to work with BLOB, CLOB, and NCLOB target columns.
Note:
This enhancement does not affect the
BFILE data because it is read-only.For the JDBC Oracle Call Interface (OCI) and Thin drivers, there is no limitation on the size of the byte array or String, and no limitation on the length specified for the stream functions, except the limits imposed by the Java language.
Note:
In Java, the array size is limited to positive Javaint or 2147483648 elements.For the server-side internal driver there is currently a limitation of 4000 bytes for operations on SQL statements, such as an INSERT statement. This limitation does not apply for PL/SQL statements. There is a simple workaround for an INSERT statement, where it is wrapped in a PL/SQL block in the following way:
BEGIN INSERT id, c INTO clob_tab VALUES(?,?); END;
You must bear in mind the following automatic switching of the input mode for large data:
There are three input modes as follows:
Direct binding
This binding is limited in size but most efficient. It places the data for all input columns inline in the block of data sent to the server. All data, including multiple executions of a batch, is sent in a single network operation.
Stream binding
This binding places data at the end. It limits batch size to one and may require multiple round trips to complete.
LOB binding
This binding creates a temporary LOB, copies data to the LOB, and binds the LOB locator. The temporary LOB is automatically freed after execution. The operation to create the temporary LOB and then to writing to the LOB requires multiple round trips. The input of the locators may be batched.
For SQL statements:
The setBytes and setBinaryStream methods use direct binding for data less than 4001 bytes.
The setBytes and setBinaryStream methods use stream binding for data larger than 4000 bytes.
In JDBC 4.0 has introduced new forms of the setAsciiStream, setBinaryStream, and setCharacterStream methods. The form, where the methods take a long argument as length, uses LOB binding for length larger than 2147483648. The form, where the length is not specified, always uses LOB binding.
The setString, setCharacterStream, and setAsciiStream methods use direct binding for data smaller than 32767 characters.
The setString, setCharacterStream, and setAsciiStream methods use stream binding for data larger than 32766 characters.
The new form of setCharacterStream method, which takes a long argument for length, uses LOB binding for length larger than 2147483647, in JDBC 4.0. The form, where the length is not specified, always uses LOB binding.
PL/SQL statements
The setBytes and setBinary stream methods use direct binding for data less than 32767 bytes.
Note:
If the underlying Database is Oracle Database release 10.x, then this data size limit is 32512 bytes, though you are working with 11g release 2 (11.2) JDBC drivers.The setBytes and setBinaryStream methods use LOB binding for data larger than 32766 bytes.
The setString, setCharacterStream, and setAsciiStream methods use direct binding for data smaller than 32767 bytes in the database character set.
Note:
If the underlying Database is Oracle Database release 10.x, then this data size limit is 32512 bytes, though you are working with 11g release 2 (11.2) JDBC drivers.The setString, setCharacterStream, and setAsciiStream methods use LOB binding for data larger than 32766 bytes in the database character set.
The automatic switching of the input mode for large data has impact on certain programs. Previously, you used to get ORA-17157 errors for attempts to use setString method for String values larger than 32766 characters. Now, depending on the type of the target parameter, an error may occur while the statement is executed or the operation may succeed.
Another impact is that the automatic switching may result in additional server-side parsing to adapt to the change in the parameter type. This would result in a performance effect, if the data sizes vary above and below the limit for repeated executions of the statement. Switching to the stream modes will effect batching as well.
The setBytesForBlob and setStringForClob methods, present in the oracle.jdbc.OraclePreparedStatement interface, use LOB binding for any data size.
The SetBigStringTryClob connection property of Oracle Database 10g Release 1 (10.1) is no longer used or needed.
The getBytes, getBinaryStream, getString, getCharacterStream, and getAsciiStream methods of ResultSet and CallableStatement are extended to work with BLOB, CLOB, and BFILE columns or OUT parameters. These methods work for any LOB of length less than 2147483648.
Note:
ThegetString and getNString methods cannot be used for retrieving BLOB column values. For more information about getNString method, refer to New Methods for National Character Set Type Data in JDK 1.6.The data interface operates by accessing the LOB locators within the driver and is transparent to application programming. It works with any supported version of the database, that is, Oracle Database 9.2 and later. For database version 11.1 or later, LOB prefetching may be used to reduce or eliminate any additional database round trips required. For more information, refer to LOB prefetching.
BLOB or CLOB data can be read and written using the same streaming mechanism as for LONG RAW and LONG data. BFILE data can be read using the same streaming mechanism. To read, use defineColumnType(nn, Types.LONGVARBINARY) or defineColumnType(nn,Types.LONGVARCHAR) method on the column. This produces a direct stream on the data as if it were a LONG RAW or LONG column. This technique is limited to Oracle Database 10g release 1 (10.1) and later.
It is a PL/SQL requirement that the Java types used as input and output for an IN OUT parameter must be the same. The automatic switching of types done by the extensions described in this chapter may cause problems with this.
Consider that you have an IN OUT CLOB parameter of a stored procedure and you wish to use setString method for setting the value for this parameter. For any IN and OUT parameter, the binds must be of the same type. The automatic switching of the input mode will cause problems unless you are sure of the data sizes. For example, if it is known that neither the input nor output data will ever be larger than 32766 bytes, then you could use setString method for the input parameter and register the OUT parameter as Types.VARCHAR and use getString method for the output parameter.
A better solution is to change the stored procedure to have separate IN and OUT parameters. That is, if you have:
CREATE PROCEDURE clob_proc( c IN OUT CLOB );
then, change it to:
CREATE PROCEDURE clob_proc( c_in IN CLOB, c_out OUT CLOB );
Another workaround is to use a container block to make the call. The clob_proc procedure can be wrapped with a Java String to use for the prepareCall statement, as follows:
"DECLARE c_temp; BEGIN c_temp := ?; clob_proc( c_temp); ? := c_temp; END;"
In either case, you may use the setString method on the first parameter and the registerOutParameter method with Types.CLOB on the second.
Be aware of the effect on the performance of the Java memory management system due to creation of very large byte array or String. Read the information provided by your Java virtual machine (JVM) vendor about the impact of very large data elements on memory management, and consider using the stream interfaces instead.
Locators are small data structures, which contain information that may be used to access the actual data of the LOB. In a database table, the locator is stored directly in the table, while the data may be in the table or in separate storage. It is common to use separate tablespaces for large LOBs.
In JDBC 4.0, LOBs should be read or written using the interfaces java.sql.Blob, java.sql.Clob, and java.sql.NClob. These provide random access to the data in the LOB.
The Oracle implementation classes oracle.sql.BLOB, oracle.sql.CLOB, and oracle.sql.NCLOB store the locator and access the data with it. The oracle.sql.BLOB and oracle.sql.CLOB classes implement the java.sql.Blob and java.sql.Clob interfaces respectively. In ojdbc6.jar, oracle.sql.NCLOB implements java.sql.NClob, but in ojdbc5.jar, it implements the java.sql.Clob interface.
In Oracle Database 11g, the Oracle JDBC drivers support the JDBC 4.0 java.sql.NClob interface in ojdbc6.jar, which is compiled with JDK 1.6 and must be used with JRE 6 or greater. The drivers support the JDBC 3.0 standard in ojdbc5.jar, which must be used with JRE 5 or greater.
In contrast, oracle.sql.BFILE is an Oracle extension, without a corresponding java.sql interface.
Certain Oracle extensions, such as some of the read and write methods present in the oracle.sql.CLOB and oracle.sql.BLOB interfaces, in earlier Oracle Database releases are no longer necessary and are deprecated. You should port your application to the standard JDBC 3.0 interface or to JDBC 4.0 interface, if you are using JDK 1.6 or above, and ojdbc6.jar.
See Also:
The Javadoc for more detailsFor Oracle Database 11g Release 2 (11.2) JDBC drivers, the number of round trips is reduced by prefetching the metadata such as the LOB length and the chunk size as well as the beginning of the LOB data along with the locator during regular fetch operations. If you select LOB columns into a result set, a new capability in the server and JDBC drivers allow some or all of the data to be prefetched to the client, when the locator is fetched. Subsequent read API calls will get the data from the prefetch buffers without any need to make database round trips.
The prefetch size is specified in bytes for BLOBs and in characters for CLOBs. It can be specified by setting the connection property oracle.jdbc.defaultLobPrefetchSize. The value of this property can be overridden in the following two ways:
At the statement level: by using the oracle.jdbc.OracleStatement.setLobPrefetchSize(int) method
At the column level: by using the form of defineColumnType method that takes length as argument
The default prefetch size is 4000.
Note:
Be aware of the possible memory consumption while setting large LOB prefetch sizes in combination with a large row prefetch size and a large number of LOB columns.See Also:
The Javadoc for more detailsStarting from Oracle Database 11g Release 1 (11.1), there is a new interface java.sql.NClob. The Oracle drivers implement the oracle.sql.NCLOB interface in both ojdbc5.jar and ojdbc6.jar. In ojdbc6.jar, it is declared to implement java.sql.NClob, whereas in ojdbc5.jar, it only extends the oracle.sql.CLOB interface.
The Oracle drivers implement the new factory methods, createBlob, createClob, and createNClob in the java.sql.Connection interface to create temporary LOBs.
Starting from JDK 1.6, the java.sql.Blob, java.sql.Clob, and java.sql.NClob interfaces have a new method free to free an LOB and release the associated resources. The Oracle drivers use this method to free an LOB, if it is a temporary LOB.
You can use temporary LOBs to store transient data. The data is stored in temporary table space rather than regular table space. You should free temporary LOBs after you no longer need them. If you do not, then the space the LOB consumes in temporary table space will not be reclaimed.
You can insert temporary LOBs into a table. When you do this, a permanent copy of the LOB is created and stored.
Note:
Inserting a temporary LOB may be preferable in some situations. For example, when the LOB data is relatively small and the overhead of copying the data is less than the cost of a database round trip to retrieve the empty locator. Remember that the data is initially stored in the temporary table space on the server and then moved into permanent storage.You create a temporary LOB with the static method createTemporary, defined in both the oracle.sql.BLOB and oracle.sql.CLOB classes. You free a temporary LOB with the freeTemporary method.
You can also create a temporary LOB by using the connection factory methods available in JDBC 4.0. For more information, refer to "LOB Creation".
You can test whether a LOB is temporary or not by calling the isTemporary method. If the LOB was created by calling the createTemporary method, then the isTemporary method returns true, else it returns false.
You can free a temporary LOB by calling the freeTemporary method. Free any temporary LOBs before ending the session or call.
Notes:
If you do not free a temporary LOB, then it will make the storage used by that LOB in the database unavailable. Frequent failure to free temporary LOBs will result in filling up temporary table space with unavailable LOB storage.
When fetching data from a ReultSet with columns that are temporary LOBs, use getClob or getBlob methods instead of getString or getBytes.
The JDBC 4.0 method free, present in java.sql.Blob, java.sql.Clob, and java.sql.NClob interfaces, supercedes the freeTemporary method.
Creating Temporary NCLOBs in JDK 1.5
You create temporary national character large objects (NCLOBs) using a variant of the createTemporary method.
Creating Temporary NCLOBs in JDK 1.6
JDBC 4.0 supports NCLOBs directly. You can use the standard factory method of java.sql.Connection interface to create an NCLOB.
This section discusses how to open and close your LOBs. The JDBC implementation of this functionality is provided using the following methods of oracle.sql.BLOB and oracle.sql.CLOB interfaces:
Note:
You do not have to necessarily open and close your LOBs. You may choose to open and close them for performance reasons.See Also:
Oracle Database SecureFiles and Large Objects Developer's Guidevoid open (int mode)
void close()
boolean isOpen()
If you do not wrap LOB operations inside an Open/Close call operation, then each modification to the LOB will implicitly open and close the LOB, thereby firing any triggers on a domain index. Note that in this case, any domain indexes on the LOB will become updated as soon as LOB modifications are made. Therefore, domain LOB indexes are always valid and may be used at any time within the same transaction.
If you wrap your LOB operations inside the Open/Close call operation, then triggers will not be fired for each LOB modification. Instead, the trigger on domain indexes will be fired at the Close call. For example, you might design your application so that domain indexes are not be updated until you call the close method. However, this means that any domain indexes on the LOB will not be valid in-between the Open/Close calls.
You open a LOB by calling the open or open(int) method. You may then read and write the LOB without any triggers associated with that LOB firing. When you finish accessing the LOB, close the LOB by calling the close method. When you close the LOB, any triggers associated with the LOB will fire.
You can check if a LOB is open or closed by calling the isOpen method. If you open the LOB by calling the open(int) method, then the value of the argument must be either MODE_READONLY or MODE_READWRITE, as defined in the oracle.sql.BLOB and oracle.sql.CLOB classes. If you open the LOB with MODE_READONLY, then any attempt to write to the LOB will result in a SQL exception.
Note:
An error occurs if you commit the transaction before closing all LOBs that were opened by the transaction. The openness of the open LOBs is discarded, but the transaction is successfully committed. Hence, all the changes made to the LOB and non-LOB data in the transaction are committed, but the triggers for domain indexing are not fixed.
The open and close methods apply only to persistent LOBs. The close method is not similar to the free or freeTemporary methods used for temporary LOBs. The free and freeTemporary methods release storage and make a LOB unusable. On the other hand, the close method indicates to the database that a modification on a LOB is complete, and triggers should be fired and indexes should be updated. A LOB is still usable after a call to the close method.
This section describes how to read data from BFILEs, using file locators. This section covers the following topics:
The BFILE data type and oracle.sql.BFILE classes are Oracle proprietary. So, there is no standard interface for them. You must use Oracle extensions for this type of data.
If you have a standard JDBC result set or callable statement object that includes BFILE locators, then you can access the locators by using the standard result set getObject method. This method returns an oracle.sql.BFILE object.
You can also access the locators by casting your result set to OracleResultSet or your callable statement to OracleCallableStatement and using the getOracleObject or getBFILE method.
Note:
If you are usinggetObject or getOracleObject methods, then remember to cast the output, as necessary.Once you have a locator, you can access the BFILE data via the API in oracle.sql.BFILE. These APIs are similar to the read methods of the java.sql.BLOB interface.
You cannot write data to the contents of the BFILE, but you can use an instance of oracle.sql.BFILE as input to a SQL statement or to a PL/SQL procedure. You can achieve this by performing one of the following:
Use the standard setObject method.
Cast the statement to OraclePreparedStatement or OracleCallableStatement, and use the setOracleObject or setBFILE method. These methods take the parameter index and an oracle.sql.BFILE object as input.
Note:
There is no standard java.sql interface for BFILEs.
Use the getBFILE methods in the OracleResultSet and OracleCallableStatement classes to retrieve an oracle.sql.BFILE object. The setBFILE methods in OraclePreparedStatement and OracleCallableStatement interfaces accept oracle.sql.BFILE object as an argument. Use these methods to write to a BFILE.
Oracle recommends that you use the getBFILE, setBFILE, and updateBFILE methods instead of the getBfile, setBfile, and updateBfile methods. For example, use the setBFILE method instead of the setBfile method.
BFILEs are read-only. The body of the data resides in the operating system (OS) file system and can be written to using only OS tools and commands. You can create a BFILE for an existing external file by executing the appropriate SQL statement either from JDBC or by using any other way to execute SQL. However, you cannot create an OS file that a BFILE would refer to by SQL or JDBC. Those are created only externally by a process that has access to server file systems.
Note:
The code examples present in this chapter, in the earlier versions of this guide, have been removed in favor of references to the sample code available for download on OTN.
You can download the demo.zip file from the following link for complete working programs:
http://www.oracle.com/technology/tech/java/sqlj_jdbc/index.html
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