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Andorid之用ConditionVariable實(shí)現(xiàn)線(xiàn)程同步
一、學(xué)習(xí)ConditionVariable之前的復(fù)習(xí)
如果你不懂wait()、notify()怎么使用，最好先復(fù)習(xí)下我之前的這篇博客，怎么使用wait()、notify()實(shí)現(xiàn)生產(chǎn)者和消費(fèi)者的關(guān)系
java之wait()、notify()實(shí)現(xiàn)非阻塞的生產(chǎn)者和消費(fèi)者
二、看下ConditionVariable源代碼實(shí)現(xiàn)
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、我們分析怎么使用
比如有多個(gè)線(xiàn)程需要執(zhí)行同樣的代碼的時(shí)候，我們一般希望當(dāng)一個(gè)線(xiàn)程執(zhí)行到這里之后，后面的線(xiàn)程在后面排隊(duì)，然后等之前的線(xiàn)程執(zhí)行完了再讓這個(gè)線(xiàn)程執(zhí)行，我們一般用synchronized實(shí)現(xiàn)，但是這里我們也可以用ConditionVariable實(shí)現(xiàn)，從源碼可以看到，我們初始化可以傳遞一個(gè)boolean類(lèi)型的參數(shù)進(jìn)去，我們可以傳遞true進(jìn)去
public ConditionVariable(boolean state)
{
mCondition = state;
}
然后你看下ConditionVariable類(lèi)里面這個(gè)方法
public void block()
{
synchronized (this) {
while (!mCondition) {
try {
this.wait();
}
catch (InterruptedException e) {
}
}
}
}
如果第一次初始化的時(shí)候mCondition是true,那么第一次調(diào)用這里就不會(huì)走到wait函數(shù)，然后我們應(yīng)該需要一個(gè)開(kāi)關(guān)讓mCondition變成false,讓第二個(gè)線(xiàn)程進(jìn)來(lái)的時(shí)候我們應(yīng)該讓線(xiàn)程執(zhí)行wait()方法，阻塞在這里，這不看下ConditionVariable類(lèi)里面這個(gè)函數(shù)
public void close()
{
synchronized (this) {
mCondition = false;
}
}
這不恰好是我們需要的，我們可以馬上調(diào)用這個(gè)函數(shù)close(),然后讓程序執(zhí)行我們想執(zhí)行的代碼，最后要記得調(diào)用open方法，如下
public void open()
{
synchronized (this) {
boolean old = mCondition;
mCondition = true;
if (!old) {
this.notifyAll();
}
}
}
因?yàn)檫@里調(diào)用了notifyAll方法，把之前需要等待的線(xiàn)程呼喚醒
所以我們使用可以這樣使用
1、初始化
ConditionVariable mLock = new ConditionVariable(true);
2、同步的地方這樣使用
mLock.block();
mLock.close();
/**
你的代碼
**/
mLock.open();
四、測(cè)試代碼分析
我先給出一個(gè)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;
}
}
}
}
運(yùn)行的結(jié)果是這樣的：
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
很明顯不是我們想要的結(jié)果，因?yàn)槲蚁胍粋€(gè)線(xiàn)程進(jìn)來(lái)了，需要等到執(zhí)行完了才讓另外一個(gè)線(xiàn)程才能進(jìn)來(lái)
我們用ConditionVariable來(lái)實(shí)現(xiàn)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();
}
}
}
運(yùn)行的結(jié)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
很明顯這是我想要的效果，還有其它辦法嗎？當(dāng)然有
我們還可以使用ReentrantLock重入鎖，代碼修改如下
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();
}
}
}
運(yùn)行的結(jié)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
很明顯這是我想要的效果，還有其它辦法嗎？當(dāng)然有，那就是用synchronized同步塊，代碼改成如下
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;
}
}
}
}
}
運(yùn)行的結(jié)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
很明顯這是我想要的效果
五、總結(jié)
在Android開(kāi)發(fā)里面我們一般實(shí)現(xiàn)線(xiàn)程通過(guò)可以用ConditionVariable、ReentrantLock(重入鎖)、synchronized、阻塞隊(duì)列(ArrayBlockingQueue、LinkedBlockingQueue)
put(E e) : 在隊(duì)尾添加一個(gè)元素，如果隊(duì)列滿(mǎn)則阻塞
size() : 返回隊(duì)列中的元素個(gè)數(shù)
take() : 移除并返回隊(duì)頭元素，如果隊(duì)列空則阻塞
作者：chen.yu
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