同步点是什么位置？

为什么不直接使用 mConnectedThread.write（下）？

在code段是从BluetoothChat样品为Android （这里找到）

  / **
 *写的ConnectedThread在非同步的方式
 *参数了要写入的字节
 * @see ConnectedThread＃写入（字节[]）
 * /
公共无效的write（byte []的输出）{
    //创建临时对象
    ConnectedThreadř;
    //该ConnectedThread副本同步
    同步（本）{
        如果（mState = STATE_CONNECTED！）回报;
        R = mConnectedThread;
    }
    //执行写操作不同步
    r.write（出）;
}
 

解决方案

需要同步，以确保你没有不一致的状态。

如果没有同步，code将是：

 公共无效的write（byte []的输出）{
    如果（mState = STATE_CONNECTED！）回报;
    mConnectedThread.write（出）;
}
 

现在，如果连接在那里关闭if语句检查和方法调用之间， mConnectedThread 可能会被分配到空，方法调用执行之前。这将导致成 NullPointerException异常。

What is the point of the synchronization here?

Why not just use mConnectedThread.write(out)?

The code snippet is from the BluetoothChat sample for Android (found here)

    /**
 * Write to the ConnectedThread in an unsynchronized manner
 * @param out The bytes to write
 * @see ConnectedThread#write(byte[])
 */
public void write(byte[] out) {
    // Create temporary object
    ConnectedThread r;
    // Synchronize a copy of the ConnectedThread
    synchronized (this) {
        if (mState != STATE_CONNECTED) return;
        r = mConnectedThread;
    }
    // Perform the write unsynchronized
    r.write(out);
}

Synchronization is needed to ensure that you don't have inconsistent state.

Without the synchronization, the code would be:

public void write(byte[] out) {
    if (mState != STATE_CONNECTED) return;
    mConnectedThread.write(out);
}

Now, if the connection where to close between the if statement check and the method invocation, mConnectedThread might be assigned to null, before the method invocation execution. That would result into a NullPointerException.