Question

我正在分析FileOutputStream。

的API

getChannel()方法可以返回null吗？如果是这样，在什么情况下？

Answer 1

FileOutputStream getChannel()方法代码

 public FileChannel getChannel() {
    synchronized (this) {
        if (channel == null) {
           channel = FileChannelImpl.open(fd, false, true, this, append);

           /*
             * Increment fd's use count. Invoking the channel's close()
            * method will result in decrementing the use count set for
            * the channel.
             */
            fd.incrementAndGetUseCount();
        }
        return channel;
    }
}

调用FileChannelImpl.open()，此代码始终创建一个新对象

public static FileChannel open(FileDescriptor fd,
                                boolean readable, boolean writable,
                                Object parent, boolean append)
 {
    return new FileChannelImpl(fd, readable, writable, parent, append);
 }

Answer 2

public FileChannel getChannel() {
    synchronized (this) {
        if (channel == null) {
            channel = FileChannelImpl.open(fd, false, true, append, this);
            fd.incrementAndGetUseCount();
        }
        return channel;
    }
}

显示无法返回null。

Answer 3

查看来源 FileChannelImpl#open

方法Open()
return new FileChannelImpl(...)
所以它创建了新的引用，它不能是null

Answer 4

来自Java Source：

    /**
 * Returns the unique {@link java.nio.channels.FileChannel FileChannel}
 * object associated with this file output stream. </p>
 *
 * <p> The initial {@link java.nio.channels.FileChannel#position()
 * </code>position<code>} of the returned channel will be equal to the
 * number of bytes written to the file so far unless this stream is in
 * append mode, in which case it will be equal to the size of the file.
 * Writing bytes to this stream will increment the channel's position
 * accordingly.  Changing the channel's position, either explicitly or by
 * writing, will change this stream's file position.
 *
 * @return  the file channel associated with this file output stream
 *
 * @since 1.4
 * @spec JSR-51
 */
public FileChannel getChannel() {
    synchronized (this) {
        if (channel == null) {
            channel = FileChannelImpl.open(fd, false, true, append, this);

            /*
             * Increment fd's use count. Invoking the channel's close()
             * method will result in decrementing the use count set for
             * the channel.
             */
            fd.incrementAndGetUseCount();
        }
        return channel;
    }
}

因此无法返回null。

可能FileOutputStream.getChannel（）返回null吗？

4 个答案: