Trail: JDBC(TM) Database Access
Lesson: JDBC Basics

使用大对象

An important feature of Blob, Clob, and NClob Java objects is that you can manipulate them without having to bring all of their data from the database server to your client computer. Some implementations represent an instance of these types with a locator (logical pointer) to the object in the database that the instance represents. Because a BLOB, CLOB, or NCLOB SQL object may be very large, the use of locators can make performance significantly faster. However, other implementations fully materialize large objects on the client computer.

If you want to bring the data of a BLOB, CLOB, or NCLOB SQL value to the client computer, use methods in the Blob, Clob, and NClob Java interfaces that are provided for this purpose. These large object type objects materialize the data of the objects they represent as a stream.

涵盖以下主题：

Adding Large Object Type Objects to Databases
Retrieving CLOB Values
Adding and Retrieving BLOB Objects
Releasing Resources Held by Large Objects

Adding Large Object Type Object to Database

The following excerpt from ClobSample.addRowToCoffeeDescriptions adds a CLOB SQL value to the table COFFEE_DESCRIPTIONS. The Clob Java object myClob contains the contents of the file specified by fileName.

public void addRowToCoffeeDescriptions(
    String coffeeName, String fileName)
    throws SQLException {

    PreparedStatement pstmt = null;
    try {
        Clob myClob = this.con.createClob();
        Writer clobWriter = myClob.setCharacterStream(1);
        String str = this.readFile(fileName, clobWriter);
        System.out.println("Wrote the following: " +
            clobWriter.toString());

        if (this.settings.dbms.equals("mysql")) {
            System.out.println(
                "MySQL, setting String in Clob " +
                "object with setString method");
            myClob.setString(1, str);
        }
        System.out.println("Length of Clob: " + myClob.length());

        String sql = "INSERT INTO COFFEE_DESCRIPTIONS " +
                     "VALUES(?,?)";

        pstmt = this.con.prepareStatement(sql);
        pstmt.setString(1, coffeeName);
        pstmt.setClob(2, myClob);
        pstmt.executeUpdate();
    } catch (SQLException sqlex) {
        JDBCTutorialUtilities.printSQLException(sqlex);
    } catch (Exception ex) {
      System.out.println("Unexpected exception: " + ex.toString());
    } finally {
        if (pstmt != null)pstmt.close();
    }
}

The following line creates a Clob Java object:

Clob myClob = this.con.createClob();

The following line retrieves a stream (in this case a Writer object named clobWriter) that is used to write a stream of characters to the Clob Java object myClob. The method ClobSample.readFile writes this stream of characters; the stream is from the file specified by the String fileName. The method argument 1 indicates that the Writer object will start writing the stream of characters at the beginning of the Clob value:

Writer clobWriter = myClob.setCharacterStream(1);

The ClobSample.readFile method reads the file line-by-line specified by the file fileName and writes it to the Writer object specified by writerArg:

private String readFile(String fileName, Writer writerArg)
        throws FileNotFoundException, IOException {

    BufferedReader br = new BufferedReader(new FileReader(fileName));
    String nextLine = "";
    StringBuffer sb = new StringBuffer();
    while ((nextLine = br.readLine()) != null) {
        System.out.println("Writing: " + nextLine);
        writerArg.write(nextLine);
        sb.append(nextLine);
    }
    // Convert the content into to a string
    String clobData = sb.toString();

    // Return the data.
    return clobData;
}

The following excerpt creates a PreparedStatement object pstmt that inserts the Clob Java object myClob into COFFEE_DESCRIPTIONS:

PreparedStatement pstmt = null;
// ...
String sql = "INSERT INTO COFFEE_DESCRIPTIONS VALUES(?,?)";
pstmt = this.con.prepareStatement(sql);
pstmt.setString(1, coffeeName);
pstmt.setClob(2, myClob);
pstmt.executeUpdate();

Retrieving CLOB Values

The method ClobSample.retrieveExcerpt retrieves the CLOB SQL value stored in the COF_DESC column of COFFEE_DESCRIPTIONS from the row whose column value COF_NAME is equal to the String value specified by the coffeeName parameter:

public String retrieveExcerpt(String coffeeName, int numChar)
    throws SQLException {

    String description = null;
    Clob myClob = null;
    PreparedStatement pstmt = null;

    try {
        String sql =
            "select COF_DESC " +
            "from COFFEE_DESCRIPTIONS " +
            "where COF_NAME = ?";

        pstmt = this.con.prepareStatement(sql);
        pstmt.setString(1, coffeeName);
        ResultSet rs = pstmt.executeQuery();

        if (rs.next()) {
            myClob = rs.getClob(1);
            System.out.println("Length of retrieved Clob: " +
                myClob.length());
        }
        description = myClob.getSubString(1, numChar);
    } catch (SQLException sqlex) {
        JDBCTutorialUtilities.printSQLException(sqlex);
    } catch (Exception ex) {
        System.out.println("Unexpected exception: " + ex.toString());
    } finally {
        if (pstmt != null) pstmt.close();
    }
    return description;
}

The following line retrieves the Clob Java value from the ResultSet object rs:

myClob = rs.getClob(1);

The following line retrieves a substring from the myClob object. The substring begins at the first character of the value of myClob and has up to the number of consecutive characters specified in numChar, where numChar is an integer.

description = myClob.getSubString(1, numChar);

Adding and Retrieving BLOB Objects

Adding and retrieving BLOB SQL objects is similar to adding and retrieving CLOB SQL objects. Use the Blob.setBinaryStream method to retrieve an OutputStream object to write the BLOB SQL value that the Blob Java object (which called the method) represents.

Releasing Resources Held by Large Objects

Blob, Clob, and NClob Java objects remain valid for at least the duration of the transaction in which they are created. This could potentially result in an application running out of resources during a long running transaction. Applications may release Blob, Clob, and NClob resources by invoking their free method.

In the following excerpt, the method Clob.free is called to release the resources held for a previously created Clob object:

Clob aClob = con.createClob();
int numWritten = aClob.setString(1, val);
aClob.free();