Java同步工具类总结

本文可作为传智播客《张孝祥-Java多线程与并发库高级应用》的学习笔记。

Semaphore

这个东西和之前的synchronized干的事差不多。

synchronized保证了,我管理的那部分代码同一时刻只有一个线程能访问

Semaphore保证了,我管理的那部分代码同一时刻最多可以有n个线程访问

package cn.itcast.heima2;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

public class SemaphoreTest {
public static void main(String[] args) {
ExecutorService service = Executors.newCachedThreadPool();
final  Semaphore sp = new Semaphore(3);
for(int i=0;i<10;i++){
Runnable runnable = new Runnable(){
public void run(){
try {
sp.acquire();
} catch (InterruptedException e1) {
e1.printStackTrace();
}
System.out.println("线程" + Thread.currentThread().getName() +
"进入，当前已有" + (3-sp.availablePermits()) + "个并发");
try {
Thread.sleep((long)(Math.random()*10000));
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("线程" + Thread.currentThread().getName() +
"即将离开");
sp.release();
//下面代码有时候执行不准确，因为其没有和上面的代码合成原子单元
System.out.println("线程" + Thread.currentThread().getName() +
"已离开，当前已有" + (3-sp.availablePermits()) + "个并发");
}
};
service.execute(runnable);
}
}

}

CycleBarrier

CycleBarrier 能做到让n个线程互相等待,当n个线程都做到某一步后,再继续下一步。
例如下面的例子,5个人去旅游,设置abc三个中途节点,所有人都到达a之后在继续走向b,所有人都到达b,然后才继续走向c。

package cn.itcast.heima2;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class CyclicBarrierTest {

public static void main(String[] args) {
ExecutorService service = Executors.newCachedThreadPool();
final  CyclicBarrier cb = new CyclicBarrier(3);
for(int i=0;i<3;i++){
Runnable runnable = new Runnable(){
public void run(){
try {
Thread.sleep((long)(Math.random()*10000));
System.out.println("线程" + Thread.currentThread().getName() +
"即将到达集合地点1，当前已有" + (cb.getNumberWaiting()+1) + "个已经到达，" + (cb.getNumberWaiting()==2?"都到齐了，继续走啊":"正在等候"));
cb.await();

Thread.sleep((long)(Math.random()*10000));
System.out.println("线程" + Thread.currentThread().getName() +
"即将到达集合地点2，当前已有" + (cb.getNumberWaiting()+1) + "个已经到达，" + (cb.getNumberWaiting()==2?"都到齐了，继续走啊":"正在等候"));
cb.await();
Thread.sleep((long)(Math.random()*10000));
System.out.println("线程" + Thread.currentThread().getName() +
"即将到达集合地点3，当前已有" + (cb.getNumberWaiting() + 1) + "个已经到达，" + (cb.getNumberWaiting()==2?"都到齐了，继续走啊":"正在等候"));
cb.await();
} catch (Exception e) {
e.printStackTrace();
}
}
};
service.execute(runnable);
}
service.shutdown();
}
}

CountDownLatch 

它保证了什么功能呢?其实和CycliBarrier也类似。
看下面这个图



这就是CycleBarrier,线程自己管理自己,大家看到人都到齐了,才继续走。

这个是CountDownLatch,由他人来协调进度。



例如跑步的时候,有个裁判,等所有的人都到齐了,他吹哨,然后大家开始跑,等所有人都跑完了,他才公布成绩。

package cn.itcast.heima2;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class CountdownLatchTest {

public static void main(String[] args) {
ExecutorService service = Executors.newCachedThreadPool();
final CountDownLatch cdOrder = new CountDownLatch(1);
final CountDownLatch cdAnswer = new CountDownLatch(3);
for(int i=0;i<3;i++){
Runnable runnable = new Runnable(){
public void run(){
try {
System.out.println("线程" + Thread.currentThread().getName() +
"正准备接受命令");
cdOrder.await();
System.out.println("线程" + Thread.currentThread().getName() +
"已接受命令");
Thread.sleep((long)(Math.random()*10000));
System.out.println("线程" + Thread.currentThread().getName() +
"回应命令处理结果");
cdAnswer.countDown();
} catch (Exception e) {
e.printStackTrace();
}
}
};
service.execute(runnable);
}
try {
Thread.sleep((long)(Math.random()*10000));

System.out.println("线程" + Thread.currentThread().getName() +
"即将发布命令");
cdOrder.countDown();
System.out.println("线程" + Thread.currentThread().getName() +
"已发送命令，正在等待结果");
cdAnswer.await();
System.out.println("线程" + Thread.currentThread().getName() +
"已收到所有响应结果");
} catch (Exception e) {
e.printStackTrace();
}
service.shutdown();

}
}

CountDownLatch里面有个计数器,初始值就是new countdownlatch时传入的

wait方法会一直等待,直到计数器的值变为0

coutdown方法可以让计数器的值减一

Exchange

A线程有数据1,它需要与B线程的数据2做交换

B线程有数据2,它需要与A线程的数据1做交换
那么什么时候交换呢?得等AB都做好准备才行。

package cn.itcast.heima2;
import java.util.concurrent.Exchanger;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ExchangerTest {

public static void main(String[] args) {
ExecutorService service = Executors.newCachedThreadPool();
final Exchanger<String> exchanger = new Exchanger<String>();
service.execute(new Runnable(){
public void run() {
try {

String data1 = "zxx";
System.out.println("线程" + Thread.currentThread().getName() +
"正在把数据" + data1 +"换出去");
Thread.sleep((long)(Math.random()*10000));
String data2 = (String)exchanger.exchange(data1);
System.out.println("线程" + Thread.currentThread().getName() +
"换回的数据为" + data2);
}catch(Exception e){

}
}
});
service.execute(new Runnable(){
public void run() {
try {

String data1 = "lhm";
System.out.println("线程" + Thread.currentThread().getName() +
"正在把数据" + data1 +"换出去");
Thread.sleep((long)(Math.random()*10000));
String data2 = (String)exchanger.exchange(data1);
System.out.println("线程" + Thread.currentThread().getName() +
"换回的数据为" + data2);
}catch(Exception e){

}
}
});
}
}

////////////////////////////////////////////////////////////////

以下为2016-9-1日更新

这里补充一些JUC包下常见类的实现

线程池 内部有worker,worker就是一个个车,runnable就是一个个人,每个worker会包装人 

参考资料http://blog.csdn.net/wojiaolinaaa/article/details/51345789

CycleBarrier

调用了ReentrantLock的lock

Semaphore于CountDownLatch 使用了AbstractQueuedSynchronizer

阻塞队列使用的也是ReentrantLock的lock

ArrayBlockingQueue内部是一个数组,但是可以循环,怎么循环呢?takeindex与putindex是两个索引,到数组的长度后,就再变成0

future是用locksupport的park函数

以上为2016-9-1日更新

///////////////////////////////////////////////////////////////