Android之AsyncTask源码分析（第五篇：execute方法只能执行一次的原因）

0、当你调用AsyncTask对象的execute（）方法时，突然发生崩溃………………内心充满了不解，它抛出异常：java.lang.IllegalStateException: Cannot execute task: the task has already been executed (a task can be executed only once)

为什么会这样呢？？？？？？？？？？？？

07-09 23:30:11.085 23377-23377/com.xx.cheez E/AndroidRuntime: FATAL EXCEPTION: main    Process: com.xx.cheez, PID: 23377    java.lang.IllegalStateException: Cannot execute task: the task has already been executed (a task can be executed only once)        at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:640)        at android.os.AsyncTask.execute(AsyncTask.java:595)        at com.wp.cheez.activity.FillSpaceActivity$MyBtnClickLis.onClick(FillSpaceActivity.java:60)        at android.view.View.performClick(View.java:6891)        at android.widget.TextView.performClick(TextView.java:12651)        at android.view.View$PerformClick.run(View.java:26083)        at android.os.Handler.handleCallback(Handler.java:789)        at android.os.Handler.dispatchMessage(Handler.java:98)        at android.os.Looper.loop(Looper.java:164)        at android.app.ActivityThread.main(ActivityThread.java:6938)        at java.lang.reflect.Method.invoke(Native Method)        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)

1、从上面的线程堆栈找到AsyncTask的源码

    @MainThread    public final AsyncTask execute(Params... params) {        return executeOnExecutor(sDefaultExecutor, params); //此处为595行    }

上面的堆栈明确说明execute方法位于AsyncTask类的595行

2、595行执行的是executeOnExecutor方法,让我们去看看executeOnExecutor方法里面又干了什么？在executeOnExecutor方法中找到了抛出异常对象的地方，请看下面代码中的注释

    @MainThread    public final AsyncTask executeOnExecutor(Executor exec,            Params... params) {        if (mStatus != Status.PENDING) {            switch (mStatus) {                case RUNNING:                    throw new IllegalStateException("Cannot execute task:"                            + " the task is already running."); //找到了抛出异常的地方                case FINISHED:                    throw new IllegalStateException("Cannot execute task:"                            + " the task has already been executed "                            + "(a task can be executed only once)");            }        }        mStatus = Status.RUNNING;        onPreExecute();   //看到老朋友了是不是，模版方法模式用到了极致        mWorker.mParams = params;        exec.execute(mFuture);        return this;    }

3、发现AsyncTask的mStatus == RUNNING的时候，executeOnExecutor（）方法会抛出java.lang.IllegalStateException: Cannot execute task: the task has already been executed..……此处省略好多字，让我们再看下关键代码，注意注释

if (mStatus != Status.PENDING) {            switch (mStatus) {                case RUNNING:                    throw new IllegalStateException("Cannot execute task:"                            + " the task is already running.");  //…………这里…………                case FINISHED:                    throw new IllegalStateException("Cannot execute task:"                            + " the task has already been executed "                            + "(a task can be executed only once)");            }        }

4、那么AsyncTask对象持有的mStatus什么时候会等于RUNNING呢？此时需要去找一下mStatus是在哪个位置被赋值为RUNNING的！！！

首先发现mStatus的初始值是Status.PENDING，而且还加了volatile，保证共享变量mStatus的可见性与有序性（一个线程写入该共享变量，另一个线程一定能读到它的最新值，并且volatile会禁止编译器与处理器的重排序）

private volatile Status mStatus = Status.PENDING;

5、那么第4条中提到的Status.PEDING又是什么呢？？？

AsyncTask类定义了一个称做Status的内部枚举类，它专门用于标记AsyncTask对象的状态，可以看到一共有三个状态

PENDING 代表准备状态

RUNNING 代表运行状态

FINISHED 代表完成状态

public enum Status {           PENDING,                RUNNING,                FINISHED,    }

6、继续寻找mStatus什么时候被赋值为RUNNING的

在整个AsyncTask类中只有一处为mStatus赋值为RUNNING，那就是在executeOnExecutor方法中……

    @MainThread    public final AsyncTask executeOnExecutor(Executor exec,            Params... params) {                ........        mStatus = Status.RUNNING;   //看这里        ........        return this;    }

7、答案明确了

一个AsyncTask对象，它的execute方法只能调用一次！再次调用execute方法时，内部调用的executeOnExecutor方法有一个判断逻辑，会根据mStatus的状态抛出一个IllegalStateException对象

当mStatus的状态为RUNNING或FINISHED时，都会抛出异常

if (mStatus != Status.PENDING) {            switch (mStatus) {                case RUNNING:                    throw new IllegalStateException("Cannot execute task:"                            + " the task is already running.");                case FINISHED:                    throw new IllegalStateException("Cannot execute task:"                            + " the task has already been executed "                            + "(a task can be executed only once)");            }        }

8、重温下mStatus在AsyncTask对象生命周期内的状态变化（下面都是mStatus仅有的赋值代码）

调用execute方法前，mStatus的状态为PENDING（AsyncTask对象创建阶段赋值）

    private volatile Status mStatus = Status.PENDING;

调用execute方法后，mStatus的状态更新为RUNNING（executeOnExecutor方法中赋值）

        mStatus = Status.RUNNING;

所有AsyncTask的任务执行完成后，mSatus的状态又变为FINISHED（finish方法中赋值）

        mStatus = Status.FINISHED;

7、解决办法

想再执行一个任务怎么办？？

答：再new一个AsyncTask对象，再次调用execute方法……

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