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
