Andorid之用ConditionVariable实现线程同步

一、学习ConditionVariable之前的复习

如果你不懂wait()、notify()怎么使用，最好先复习下我之前的这篇博客，怎么使用wait()、notify()实现生产者和消费者的关系

java之wait()、notify()实现非阻塞的生产者和消费者

二、看下ConditionVariable源代码实现

package android.os;/*** Class that implements the condition variable locking paradigm.** <p>* This differs from the built-in java.lang.Object wait() and notify()* in that this class contains the condition to wait on itself.  That means* open(), close() and block() are sticky.  If open() is called before block(),* block() will not block, and instead return immediately.** <p>* This class uses itself as the object to wait on, so if you wait()* or notify() on a ConditionVariable, the results are undefined.*/
public class ConditionVariable
{private volatile boolean mCondition;/*** Create the ConditionVariable in the default closed state.*/public ConditionVariable(){mCondition = false;}/*** Create the ConditionVariable with the given state.* * <p>* Pass true for opened and false for closed.*/public ConditionVariable(boolean state){mCondition = state;}/*** Open the condition, and release all threads that are blocked.** <p>* Any threads that later approach block() will not block unless close()* is called.*/public void open(){synchronized (this) {boolean old = mCondition;mCondition = true;if (!old) {this.notifyAll();}}}/*** Reset the condition to the closed state.** <p>* Any threads that call block() will block until someone calls open.*/public void close(){synchronized (this) {mCondition = false;}}/*** Block the current thread until the condition is opened.** <p>* If the condition is already opened, return immediately.*/public void block(){synchronized (this) {while (!mCondition) {try {this.wait();}catch (InterruptedException e) {}}}}/*** Block the current thread until the condition is opened or until* timeout milliseconds have passed.** <p>* If the condition is already opened, return immediately.** @param timeout the maximum time to wait in milliseconds.** @return true if the condition was opened, false if the call returns* because of the timeout.*/public boolean block(long timeout){// Object.wait(0) means wait forever, to mimic this, we just// call the other block() method in that case.  It simplifies// this code for the common case.if (timeout != 0) {synchronized (this) {long now = System.currentTimeMillis();long end = now + timeout;while (!mCondition && now < end) {try {this.wait(end-now);}catch (InterruptedException e) {}now = System.currentTimeMillis();}return mCondition;}} else {this.block();return true;}}
}

三、我们分析怎么使用

比如有多个线程需要执行同样的代码的时候，我们一般希望当一个线程执行到这里之后，后面的线程在后面排队，然后等之前的线程执行完了再让这个线程执行，我们一般用synchronized实现，但是这里我们也可以用ConditionVariable实现，从源码可以看到，我们初始化可以传递一个boolean类型的参数进去，我们可以传递true进去

  public ConditionVariable(boolean state){mCondition = state;}

然后你看下ConditionVariable类里面这个方法

    public void block(){synchronized (this) {while (!mCondition) {try {this.wait();}catch (InterruptedException e) {}}}}

如果第一次初始化的时候mCondition是true,那么第一次调用这里就不会走到wait函数，然后我们应该需要一个开关让mCondition变成false,让第二个线程进来的时候我们应该让线程执行wait()方法，阻塞在这里，这不看下ConditionVariable类里面这个函数

    public void close(){synchronized (this) {mCondition = false;}}

这不恰好是我们需要的，我们可以马上调用这个函数close(),然后让程序执行我们想执行的代码，最后要记得调用open方法，如下

   public void open(){synchronized (this) {boolean old = mCondition;mCondition = true;if (!old) {this.notifyAll();}}}

因为这里调用了notifyAll方法，把之前需要等待的线程呼唤醒

所以我们使用可以这样使用

1、初始化

ConditionVariable mLock = new ConditionVariable(true);

2、同步的地方这样使用

   mLock.block();mLock.close();/**你的代码**/mLock.open();

四、测试代码分析

我先给出一个原始Demo

public class MainActivity extends ActionBarActivity {public static final String TAG = "ConditionVariable_Test";@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);for (int i = 0; i < 10; i++) {new Mythread("" + i).start();}}public int num = 5;class Mythread extends Thread {String name;public Mythread(String name) {this.name = name;}@Overridepublic void run() {while (true) {try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}num--;if (num >= 0)Log.i(TAG, "thread name is:" + name + " num is:" + num);elsebreak;}}}
}

运行的结果是这样的：

ConditionVariable_Test  I  thread name is:0 num is:4I  thread name is:1 num is:3I  thread name is:2 num is:2I  thread name is:3 num is:1I  thread name is:4 num is:0

很明显不是我们想要的结果，因为我想一个线程进来了，需要等到执行完了才让另外一个线程才能进来

我们用ConditionVariable来实现下

package com.example.conditionvariable;import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;import android.os.Bundle;
import android.os.ConditionVariable;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;public class MainActivity extends ActionBarActivity {public static final String TAG = "ConditionVariable_Test";@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);mCondition = new ConditionVariable(true);for (int i = 0; i < 10; i++) {new Mythread("" + i).start();}}public int num = 5;class Mythread extends Thread {String name;public Mythread(String name) {this.name = name;}@Overridepublic void run() {mCondition.block();mCondition.close();while (true) {try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}num--;if (num >= 0)Log.i(TAG, "thread name is:" + name + " num is:" + num);elsebreak;}mCondition.open();}}
}

运行的结果如下

onditionVariable_Test  I  thread name is:0 num is:4I  thread name is:0 num is:3I  thread name is:0 num is:2I  thread name is:0 num is:1I  thread name is:0 num is:0

很明显这是我想要的效果，还有其它办法吗？当然有

我们还可以使用ReentrantLock重入锁，代码修改如下

public class MainActivity extends ActionBarActivity {public static final String TAG = "ConditionVariable_Test";private Lock lock = new ReentrantLock();@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);for (int i = 0; i < 10; i++) {new Mythread("" + i).start();}}public int num = 5;class Mythread extends Thread {String name;public Mythread(String name) {this.name = name;}@Overridepublic void run() {lock.lock();while (true) {try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}num--;if (num >= 0)Log.i(TAG, "thread name is:" + name + " num is:" + num);elsebreak;}lock.unlock();}}
}

运行的结果如下

onditionVariable_Test  I  thread name is:0 num is:4I  thread name is:0 num is:3I  thread name is:0 num is:2I  thread name is:0 num is:1I  thread name is:0 num is:0

很明显这是我想要的效果，还有其它办法吗？当然有，那就是用synchronized同步块，代码改成如下

package com.example.conditionvariable;import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;import android.os.Bundle;
import android.os.ConditionVariable;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;public class MainActivity extends ActionBarActivity {public static final String TAG = "ConditionVariable_Test";@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);for (int i = 0; i < 10; i++) {new Mythread("" + i).start();}}public int num = 5;class Mythread extends Thread {String name;public Mythread(String name) {this.name = name;}@Overridepublic void run() {synchronized (MainActivity.class) {while (true) {try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}num--;if (num >= 0)Log.i(TAG, "thread name is:" + name + " num is:" + num);elsebreak;}}}}
}

运行的结果如下

onditionVariable_Test  I  thread name is:0 num is:4I  thread name is:0 num is:3I  thread name is:0 num is:2I  thread name is:0 num is:1I  thread name is:0 num is:0

很明显这是我想要的效果

五、总结

在Android开发里面我们一般实现线程通过可以用ConditionVariable、ReentrantLock(重入锁)、synchronized、阻塞队列(ArrayBlockingQueue、LinkedBlockingQueue)
   put(E e) : 在队尾添加一个元素，如果队列满则阻塞
   size() : 返回队列中的元素个数
   take() : 移除并返回队头元素，如果队列空则阻塞