结束AsyncTask和Thread的有效方案

如何结束AsyncTask

上周在项目中碰见一个问题，AsyncTask调用了cancel（true）方法后任务没有取消，造成的结果就是线程继续运行把不该返回的值返回了，源码中对cancel（）的说明如下：

/**
     * 
Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when cancel is called,
     * this task should never run. If the task has already started,
     * then the mayInterruptIfRunning parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     * 
     * 
Calling this method will result in {@link #onCancelled(Object)} being
     * invoked on the UI thread after {@link #doInBackground(Object[])}
     * returns. Calling this method guarantees that {@link #onPostExecute(Object)}
     * is never invoked. After invoking this method, you should check the
     * value returned by {@link #isCancelled()} periodically from
     * {@link #doInBackground(Object[])} to finish the task as early as
     * possible.
     *
     * @param mayInterruptIfRunning true if the thread executing this
     *        task should be interrupted; otherwise, in-progress tasks are allowed
     *        to complete.
     *
     * @return false if the task could not be cancelled,
     *         typically because it has already completed normally;
     *         true otherwise
     *
     * @see #isCancelled()
     * @see #onCancelled(Object)
     */
    public final boolean cancel(boolean mayInterruptIfRunning) {
        mCancelled.set(true);
        return mFuture.cancel(mayInterruptIfRunning);
    }

大概是说AsyncTask的cancel方法被调用后并不会真正将AsyncTask任务结束，而是将isCancelled置为true。
下面是一个demo，在AsyncTaskTest.java文件中重写doInBackground()方法，在这个方法中写一个死循环；

AsyncTaskTest.java

@Override
    protected Void doInBackground(Integer... integers) {
        if (true) {
            while(true){
                integers[0]++;
                Log.i(TAG,"doInBackground is called for "+integers[0]+"times");
                try{
                    Thread.sleep(1000L);

                }catch(Exception e){

                }
            }
        }
        return null;
    }
@Override
    protected void onCancelled(Void aVoid) {
        super.onCancelled(aVoid);
        Log.i(TAG,"AsyncTask is abnormally terminated ");
    }

在MainActivity.java中执行AsyncTaskTest任务

private void init(){
        asyncTaskTest = new AsyncTaskTest();
        asyncTaskTest.execute(0);
        cancelButton = findViewById(R.id.cancelled);
        cancelButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                asyncTaskTest.cancel(true);
            }
        });
    }

点击cancelButton时调用asyncTaskTest.cancel(true)，但是Logcat还是会输出doInBackground()函数中的日志，同时onCancelled()函数也会被调用。这里证实源码中“AsyncTask的cancel方法被调用后并不会真正将AsyncTask任务结束，而是将isCancelled置为true。”的结论。

然后将AsyncTaskTest.java文件中的doInBackground()修改如下（其实就添加如果isCancelled()为true就return的逻辑）：

@Override
    protected Void doInBackground(Integer... integers) {
        if (true) {
            while(true){
                if(isCancelled()){
                    return null;
                }
                integers[0]++;
                Log.i(TAG,"doInBackground is called for "+integers[0]+"times");
                try{
                    Thread.sleep(1000L);

                }catch(Exception e){

                }
            }
        }
        return null;
    }

再去点击cancelButton后发现doInBackground()函数不会再输出日志了。所以在需要终止AsyncTask时，要在doInBackground()和onPostExecute（）中周期性去判断isCancelled的值，然后在业务层自己控制。

为什么要这么麻烦呢？调用AsyncTask中cancel（）直接结束任务就完事儿不好吗？这里就需要我们了解AsyncTask在系统中是怎么运行的，需要去看看源码，我也不知道为什么，等我研究完了再更新。

如何结束Thread

Thread的结束和AsyncTask结束的方式如出一辙，有这三个方法interrupt()、isInterrupted()这些个名字有点秀，interrupt()类似于AsyncTask中的cancel()，isInterrupted()类似于AsyncTask中的isCancelled()。