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Mapping of java.sql.Blob and java.sql.Clob interfaces
In the JDBC API, java.sql.Blob is the mapping for the SQL BLOB
(binary large object) type; java.sql.Clob is the mapping for the SQL
CLOB (character large object) type. BLOB and CLOB objects are collectively
referred to as LOBs (large objects).
The Derby
implementation of the java.sql.Blob and java.sql.Clob interfaces
is LOCATOR-based, meaning that the implementation provides a logical pointer to
a LOB rather than a complete copy of the object. Also,
Derby does not materialize a
LOB when you use the BLOB or CLOB data type. You can, however, call methods on
a java.sql.Blob and java.sql.Clob object to materialize it
(that is, to retrieve the entire object or parts of it).
Derby implements
all of the methods for these interfaces except for the setBlob,
getBlob, setClob, and getClob methods of the
CallableStatement interface.
To use the java.sql.Blob and java.sql.Clob features:
- Use the SQL BLOB type for columns which hold very large binary values.
- Use the SQL CLOB type for columns which hold very large string values.
- Use the getBlob and getClob methods of the
java.sql.ResultSet interface to retrieve a LOB using its locator.
You can then materialize all or part of the LOB by calling Blob and
Clob methods. Alternatively, you can call the ResultSet.getBytes
method to materialize a BLOB, and you can call the ResultSet.getString
method to materialize a CLOB.
Casting between strings and BLOBs is not recommended
because casting is platform- and database-dependent.
As with other character datatypes,
Derby treats CLOBs as unicode
strings and writes them to disk using UTF8 encoding. With a Java database like
Derby, you do not need to
worry about character sets and codepages.
Restrictions on BLOB and CLOB objects (LOB-types)
- LOB-types cannot be compared for equality (=) and non-equality (!=, <>).
- LOB-typed values are not orderable, so <, <=, >, >= tests
are not supported.
- LOB-types cannot be used in indices or as primary key columns.
- DISTINCT, GROUP BY, and ORDER BY clauses are also prohibited on LOB-types.
- LOB-types cannot be involved in implicit casting as other base-types.
Recommendation: Because the lifespan of a java.sql.Blob or java.sql.Clob ends
when the transaction commits, turn off auto-commit with the java.sql.Blob or java.sql.Clob features.
Table 1. Implementation Notes on java.sql.Blob Methods
| Returns |
Signature |
Implementation Notes |
| byte[] |
getBytes(long pos, int length) |
Exceptions are raised if pos < 1, if pos is
larger than the length of the , or if length <= 0. |
| long |
position(byte[] pattern, long start) |
Exceptions are raised if pattern == null, if start <
1, or if pattern is an array of length 0. |
| long |
position(Blob pattern, long start) |
Exceptions are raised if pattern == null, if start <
1, if pattern has length 0, or if an exception is thrown when trying
to read the first byte of pattern. |
Table 2. Implementation Notes on java.sql.Clob Methods
| Returns |
Signature |
Implementation Notes |
| String |
getSubString(long pos, int length) |
Exceptions are raised if pos < 1, if pos is
larger than the length of the Clob, or if length <= 0. |
| long |
position(Clob searchstr, long start) |
Exceptions are raised if searchStr == null or start <
1, if searchStr has length 0, or if an exception is thrown when trying
to read the first char of searchStr. |
| long |
position(String searchstr, long start) |
Exceptions are raised if searchStr == null or start <
1, or if the pattern is an empty string. |
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