Andorid之用ConditionVariable实现线程同步-阿里云开发者社区

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";
  @Override
  protected 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;
    }
    @Override
    public 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);
        else
          break;
      }
    }
  }
}

运行的结果是这样的：

ConditionVariable_Test I thread name is:0 num is:4

I thread name is:1 num is:3

I thread name is:2 num is:2

I thread name is:3 num is:1

I 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";
  @Override
  protected 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;
    }
    @Override
    public 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);
        else
          break;
      }
      mCondition.open();
    }
  }
}

运行的结果如下

onditionVariable_Test I thread name is:0 num is:4

I thread name is:0 num is:3

I thread name is:0 num is:2

I thread name is:0 num is:1

I 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();
  @Override
  protected 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;
    }
    @Override
    public 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);
        else
          break;
      }
      lock.unlock();
    }
  }
}

运行的结果如下

onditionVariable_Test I thread name is:0 num is:4

I thread name is:0 num is:3

I thread name is:0 num is:2

I thread name is:0 num is:1

I 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";
  @Override
  protected 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;
    }
    @Override
    public 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);
          else
            break;
        }
      }
    }
  }
}

运行的结果如下

onditionVariable_Test I thread name is:0 num is:4

I thread name is:0 num is:3

I thread name is:0 num is:2

I thread name is:0 num is:1

I 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

Andorid之用ConditionVariable实现线程同步

一、学习ConditionVariable之前的复习

二、看下ConditionVariable源代码实现

三、我们分析怎么使用

四、测试代码分析

五、总结

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