Guava并发（1）——Monitor的使用

一、说明

Monitor就像java本土的synchronized,
ReentrantLock一样，每次只运行一个线程占用，且可重占用，每一次占用会对应一次退出占用。Monitor.enter //进入Monitor块，将阻塞其他线程知道Monitor.leave
<span style="color:#FF0000;">Monitor.enterWhen </span> //进入Monitor块，将阻塞其他线程知道Monitor.leave

Monitor.tryEnter //尝试进入Monitor块，true表示可以进入, false表示不能，并且不会一直阻塞
<span style="color:#FF0000;">Monitor.tryEnterIf</span> //根据条件尝试进入Monitor块


Monitor.Guard是造成线程阻塞的条件，与Monitor配合使用。
Monitor.Guard listBelowCapacity = new
Monitor.Guard(monitor) {
@Override
public boolean isSatisfied() {
。。。。。
}
};

二、代码

package com.wll.guava.concurrent;

import com.google.common.util.concurrent.Monitor;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
* Monitor类语义和 synchronized 或者 ReentrantLocks是一样的， 只允许一个线程进入
*/
public class MonitorExample {
private static final int MAX_SIZE = 10;
private Monitor monitor = new Monitor();
private List<String> list = new ArrayList<String>();

Monitor.Guard listBelowCapacity = new
Monitor.Guard(monitor) {
@Override
public boolean isSatisfied() {
return list.size() < MAX_SIZE;
}
};

public void addToListWait(String item) throws InterruptedException {
// 超过MAX_SIZE， 会锁死（阻塞）
this.monitor.enterWhen(listBelowCapacity);
try {
list.add(item);
System.out.println("添加元素[" + item + "]成功，当前List.size=" + list.size() + "～");
} finally { // 确保线程会退出Monitor锁
monitor.leave();
}
}
public void addToListSkipWait(String item) throws InterruptedException {
// 超过MAX_SIZE， 会锁死
//this.monitor.enterWhen(listBelowCapacity);

// 超过返回false 不会锁死
Boolean isOK = monitor.tryEnterIf(listBelowCapacity);
System.out.println("Thread[" + Thread.currentThread().getName() + "] item=" + item + ",获取令牌：isOK=" + isOK);
if (isOK) {
try {
list.add(item);
System.out.println("添加元素[" + item + "]成功，当前List.size=" + list.size() + "～");
} finally { // 确保线程会退出Monitor锁
monitor.leave();
}
}
}

public static void main(String[] args) {
final MonitorExample monitorDemo = new MonitorExample();
for (int i = 0; i < 5; i++) {
new Thread() {
public void run() {
for (int count = 0; count < 6; count++) {
try {
// monitorDemo.addToListWait(count + "------------------>" + Thread.currentThread().getName());
monitorDemo.addToListSkipWait(count + "------------------>" + Thread.currentThread().getName());
Thread.sleep(100L);
} catch (Exception e) {
System.out.println(e);
}
}
}
}.start();
}

// 等待所有线程执行完毕
try {
Thread.sleep(1000L);
} catch (InterruptedException e) {
e.printStackTrace();
}

System.out.println("---------------------------- 长长的分割线 ---------------------------");
Iterator iteratorStringList = monitorDemo.list.iterator();
while (iteratorStringList.hasNext()) {
System.out.println(iteratorStringList.next());
}
}
}

三、执行结果

Connected to the target VM, address: '127.0.0.1:61207', transport: 'socket'
Thread[Thread-2] item=0------------------>Thread-2,获取令牌：isOK=false
Thread[Thread-1] item=0------------------>Thread-1,获取令牌：isOK=false
Thread[Thread-4] item=0------------------>Thread-4,获取令牌：isOK=false
Thread[Thread-0] item=0------------------>Thread-0,获取令牌：isOK=false
Thread[Thread-3] item=0------------------>Thread-3,获取令牌：isOK=true
添加元素[0------------------>Thread-3]成功，当前List.size=1～
Thread[Thread-3] item=1------------------>Thread-3,获取令牌：isOK=true
添加元素[1------------------>Thread-3]成功，当前List.size=2～
Thread[Thread-2] item=1------------------>Thread-2,获取令牌：isOK=true
Thread[Thread-0] item=1------------------>Thread-0,获取令牌：isOK=false
Thread[Thread-4] item=1------------------>Thread-4,获取令牌：isOK=false
添加元素[1------------------>Thread-2]成功，当前List.size=3～
Thread[Thread-1] item=1------------------>Thread-1,获取令牌：isOK=false
Thread[Thread-2] item=2------------------>Thread-2,获取令牌：isOK=true
添加元素[2------------------>Thread-2]成功，当前List.size=4～
Thread[Thread-0] item=2------------------>Thread-0,获取令牌：isOK=true
添加元素[2------------------>Thread-0]成功，当前List.size=5～
Thread[Thread-4] item=2------------------>Thread-4,获取令牌：isOK=true
添加元素[2------------------>Thread-4]成功，当前List.size=6～
Thread[Thread-1] item=2------------------>Thread-1,获取令牌：isOK=true
添加元素[2------------------>Thread-1]成功，当前List.size=7～
Thread[Thread-3] item=2------------------>Thread-3,获取令牌：isOK=true
添加元素[2------------------>Thread-3]成功，当前List.size=8～
Thread[Thread-3] item=3------------------>Thread-3,获取令牌：isOK=true
添加元素[3------------------>Thread-3]成功，当前List.size=9～
Thread[Thread-2] item=3------------------>Thread-2,获取令牌：isOK=false
Thread[Thread-0] item=3------------------>Thread-0,获取令牌：isOK=true
添加元素[3------------------>Thread-0]成功，当前List.size=10～
Thread[Thread-1] item=3------------------>Thread-1,获取令牌：isOK=false
Thread[Thread-4] item=3------------------>Thread-4,获取令牌：isOK=false
Thread[Thread-2] item=4------------------>Thread-2,获取令牌：isOK=false
Thread[Thread-1] item=4------------------>Thread-1,获取令牌：isOK=false
Thread[Thread-4] item=4------------------>Thread-4,获取令牌：isOK=false
Thread[Thread-3] item=4------------------>Thread-3,获取令牌：isOK=false
Thread[Thread-0] item=4------------------>Thread-0,获取令牌：isOK=false
Thread[Thread-3] item=5------------------>Thread-3,获取令牌：isOK=false
Thread[Thread-2] item=5------------------>Thread-2,获取令牌：isOK=false
Thread[Thread-4] item=5------------------>Thread-4,获取令牌：isOK=false
Thread[Thread-0] item=5------------------>Thread-0,获取令牌：isOK=false
Thread[Thread-1] item=5------------------>Thread-1,获取令牌：isOK=false
---------------------------- 长长的分割线 ---------------------------
0------------------>Thread-3
1------------------>Thread-3
1------------------>Thread-2
2------------------>Thread-2
2------------------>Thread-0
2------------------>Thread-4
2------------------>Thread-1
2------------------>Thread-3
3------------------>Thread-3
3------------------>Thread-0
Disconnected from the target VM, address: '127.0.0.1:61207', transport: 'socket'

Process finished with exit code 0
四、总结

 略。