MappedByteBuffer 之文件删除问题

熟悉nio的朋友都知道，MappedByteBuffer大幅提高了IO效率，但却有个比较严重的问题。

看如下测试代码：

public class TestMappedByteBufferDeleteFile {
	
	File testFile;
	
	/**
	 * 创建测试文件
	 * @throws URISyntaxException 
	 * @throws IOException 
	 */
	@Before public void createFile() throws URISyntaxException, IOException {
		testFile = new File(this.getClass().getResource(".").getPath() + "/test.txt");
		if(!testFile.exists()) {
			testFile.createNewFile();
		}
		FileOutputStream fos = new FileOutputStream(testFile);
		fos.write("TestMappedByteBufferDeleteFile".getBytes());
		fos.close();
	}
	
	/**
	 * 测试使用MappedByteBuffer后直接删除文件
	 * @throws IOException 
	 */
	@Test public void testDeleteFile() throws IOException {
		map(testFile);
		testFile.delete();
		Assert.assertEquals(false, testFile.exists());
	}
	
	/**
	 * 映射文件，返回MappedByteBuffer
	 * @param file
	 * @return
	 * @throws IOException
	 */
	public MappedByteBuffer map(File file) throws IOException {
		FileInputStream in = new FileInputStream(file);   
		FileChannel ch = in.getChannel();
		MappedByteBuffer buffer = ch.map(FileChannel.MapMode.READ_ONLY, 0, file.length());
		ch.close();
		in.close();
		return buffer;
	}
}

 Junit运行，结果断言失败，文件没删除成功。

 

解决方法是加上如下代码：

/**
	 * 清理MappedByteBuffer句柄
	 * @param buffer
	 */
	public static void clean(final MappedByteBuffer buffer) {
		if (buffer == null) {
			return;
		}
		AccessController.doPrivileged(new PrivilegedAction<Object>() {
			
			public Object run() {
				try {
					Method getCleanerMethod = buffer.getClass().getMethod(
							"cleaner", new Class[0]);
					if (getCleanerMethod != null) {
						getCleanerMethod.setAccessible(true);
						Object cleaner = getCleanerMethod.invoke(buffer,
								new Object[0]);
						Method cleanMethod = cleaner.getClass().getMethod(
								"clean", new Class[0]);
						if (cleanMethod != null) {
							cleanMethod.invoke(cleaner, new Object[0]);
						}
					}
				} catch (Exception e) {
					e.printStackTrace();
				}
				return null;
			}
}

 然后将testDeleteFile() 作如下修改：

/**
	 * 测试使用MappedByteBuffer后调用clean方法后删除文件
	 * @throws IOException 
	 */
	@Test public void testDeleteFile() throws IOException {
		MappedByteBuffer buffer = map(testFile);
		clean(buffer);
		testFile.delete();
		Assert.assertEquals(false, testFile.exists());
	}

 

再运行Junit，成功！

 

之所以出现这种情况，是因为文件句柄没有被清除，还被MappedByteBuffer占据，导致外部无法操作。而要清除，得等GC收集了。

 

不知Sun何时可以提供MappedByteBuffer的unmap！

另请关注该链接：http://bugs.sun.com/view_bug.do?bug_id=4724038

评论

1 楼 dyllove98 2011-05-26  

Here is a workaround. It uses internal JVM API to call the cleaner instead of reflection. I know it is uggly, but as long as Java provides this for us this is what we have to do.  Should be safe in a single thread.

Any comments?

ByteBuffer buf = null;
try {
    buf = allocateByteBuffer();
    doWork(buf);
} finally {
    if (buf != null && buf.isDirect()) {
        ((sun.nio.ch.DirectBuffer) buf).cleaner().clean();
    }
}