java.lang.UnsupportedOperationException at java.nio.ByteBuffer.array(ByteBuffer.java:959)

Question

The following Java code compiles, but there's an error at runtime:

# javac ByteBufTest.java
# java ByteBufTest
Exception in thread "main" java.lang.UnsupportedOperationException
    at java.nio.ByteBuffer.array(ByteBuffer.java:959)
    at ByteBufTest.<init>(ByteBufTest.java:12)
    at ByteBufTest.main(ByteBufTest.java:33)
#

Why does this happen?

Note：Next, I need to use mDirectBuffer in JNI, so I have to use the ByteBuffer.allocateDirect(TEST_BUFFER_SIZE) function。

ByteBufTest.java:

import java.nio.ByteBuffer;

public class ByteBufTest {

    public static final int TEST_BUFFER_SIZE = 128;

    private ByteBuffer mDirectBuffer;

    public ByteBufTest() {
        mDirectBuffer = ByteBuffer.allocateDirect(TEST_BUFFER_SIZE);
        byte[] buf = mDirectBuffer.array();
        buf[1]=100;
    }

    public void test() {

        printBuffer("nativeInitDirectBuffer",mDirectBuffer.array());

    }

    private void printBuffer( String tag, byte[] buffer ) {
        StringBuffer sBuffer = new StringBuffer();
        for( int i=0; i<buffer.length; i++ ) {
            sBuffer.append(buffer[i]);
            sBuffer.append(" ");    
        }
        //System.out.println(tag+sBuffer);
    }

    public static void main(String[] args) throws Exception {

        ByteBufTest item = new ByteBufTest();
        item.test(); 
    }
}

Kalle Richter · Accepted Answer · 2018-02-02 12:25:50Z

3

This is the expected behaviour. The Javadoc states

throws UnsupportedOperationException - If this buffer is not backed by an accessible array

You should try another approach or search for another implementation, e.g.

mDirectBuffer = ByteBuffer.wrap(new byte[TEST_BUFFER_SIZE]);

answered Feb 2, 2018 at 12:25

Kalle Richter

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2 Comments

zzkjliu Over a year ago

，I need to use mDirectBuffer in JNI, so I have to use the ByteBuffer.allocateDirect(TEST_BUFFER_SIZE) function

zzkjliu Over a year ago

For JDK，the array() method of byte buffer class returns a byte array (byte []) of the buffer’s contents. This is only valid for non-direct buffers. When used with direct buffers this method throws an exception: UnsupportedOperationException。 For android，the array() works ok。

Ankit Shubham · Accepted Answer · 2019-04-03 06:14:43Z

1

This exception occurs at runtime if the resulting buffer is not backed by an accessible array. You can try allocate() method.

answered Apr 3, 2019 at 6:14

Ankit Shubham

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1 Comment

gouessej Over a year ago

Your reply is incomplete, hasArray() exists for this purpose.

gouessej · Accepted Answer · 2022-10-27 09:00:38Z

1

You should call java.nio.ByteBuffer.hasArray() to ensure that java.nio.ByteBuffer.array() will succeed in order to write clean and portable code as stated in the Java API documentation:

If this method returns true then the array and arrayOffset methods may safely be invoked

You can allocate a direct writable NIO byte buffer in your Java source code by calling java.nio.ByteBuffer.allocateDirect(int) as you already do and call java.nio.ByteBuffer.get(byte[]) to store the content of the buffer into an array, this method is supported by Android too. Keep in mind that it's a relative operation that affects the position of the NIO buffer.

Maybe another approach would consist in using the NIO buffer as is without doing any conversion but I'm not sure that it suits your needs.

edited Oct 27, 2022 at 9:00

answered Oct 27, 2022 at 8:15

gouessej

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