在Java中将CLOB转换为String

1. Introduction

When working with databases in Java, handling large text data is a common task. Besides, the Character Large Object (CLOB) type allows databases to store extensive text data. Moreover, converting between the CLOB and String objects is often necessary when reading from or writing to databases.

In this tutorial, we’ll explore how to perform this conversion efficiently in Java.

2. Converting CLOB to String

When converting a CLOB to a String, the approach depends on the size of the CLOB.

2.1. CLOB Size <= Integer.MAX_VALUE

If the size of the CLOB is less than or equal to Integer.MAX_VALUE, we can directly use the getSubString() method to convert the CLOB to a String. This method efficiently retrieves the entire CLOB content as a String in one go:

@Test
public void givenCLOB_whenSizeLessThanMaxValue_thenConvertToString() throws SQLException {
    Clob clob = new javax.sql.rowset.serial.SerialClob("This is a sample CLOB content.".toCharArray());
    long clobLength = clob.length();

    if (clobLength <= Integer.MAX_VALUE) {
        String clobAsString = clob.getSubString(1, (int) clobLength);
        assertEquals("This is a sample CLOB content.", clobAsString);
    }
}

In this approach, the getSubString() is used to fetch the entire CLOB content directly as a String, provided the CLOB‘s length is manageable within the limits of a String object.

2.2. CLOB Size > Integer.MAX_VALUE

When the size of the CLOB exceeds the Integer.MAX_VALUE, directly converting it into a String isn’t feasible due to the memory limitations of the String class. Instead, we need to process the CLOB data in chunks to avoid memory issues. Here is an example of how we can process the CLOB with chunks:

void convertLargeCLOBtoString() throws SQLException, IOException {
    Clob clob = // Assume we have a large clob size > Integer.MAX_VALUE

    try (Reader reader = clob.getCharacterStream()) {
        char[] buffer = new char[4096];
        int charsRead;

        while ((charsRead = reader.read(buffer)) != -1) {
            processChunk(buffer, charsRead); // Process each chunk as needed
        }
    }
}

We read the CLOB data using a Reader in manageable chunks (e.g., 4096 characters at a time). Instead of trying to store all the data in a single String, we process each chunk as it’s read. This could involve writing to a file, processing the data directly, or any other operation that suits our application’s needs.

This chunk-based approach ensures that even very large CLOBs can be handled efficiently without running into memory issues that would occur if we tried to fit all the data into a single String.

3. Converting String to CLOB

Let’s dive into the implementation of how to convert a String object into a Clob object:

@Test
public void givenString_whenConvertToCLOB_thenCorrect() throws SQLException {
    String sampleText = "This is a sample text to be stored as CLOB.";
    char[] charArray = sampleText.toCharArray();
    Clob clob = new javax.sql.rowset.serial.SerialClob(charArray);

    assertEquals(sampleText, clob.getSubString(1, (int) clob.length()));
}

Here, we define a String object named sampleText containing the text we want to store as a Clob object. Next, we convert the String into a character array (charArray) using the toCharArray() method. This step prepares our text for storage in the Clob object.

Afterward, we create a Clob object using its constructor, SerialClob(charArray), where charArray represents the character data to be stored.

4. Conclusion

In this tutorial, we’ve explored how to convert between Clob objects and String representations in Java, which is essential for managing large text data when interacting with databases.

As always, the complete code samples for this article can be found over on GitHub.