4，临界区 之 synchronized

前提背景：多个并发线程共享同一个资源时，为防止这些共享资源可能出现的错误或数据不一致问题，提出了临界区的概念

临界区: 指一个用以访问共享资源的代码块，这个代码块在同一时间内只能允许一个线程访问

实现方式：synchronized 或 lock

synchronized 方式如下， User 给addMoney方法加上了synchronized , 相当于加了锁，后面同时起了3个线程，都调这个方法，因为设置了sleep 3 秒，可以看到 会有2个线程处于 BLOCKED 的状态,直到释放

package com.zyguo.thread;

public class User {
private int money;
private int id;

public User( int id ){
this.setId(id);
}
public int getMoney() {
return money;
}

public synchronized int addMoney( int incrNum ){
this.money = this.money + incrNum;
try {
Thread.sleep( 3000 );
System.out.println("addMoney, currThread=" + Thread.currentThread() );
} catch (InterruptedException e) {
e.printStackTrace();
}
return this.money;
}

public synchronized int reducMoney( int reducNum ){
this.money = this.money - reducNum;
return this.money;
}

public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
}

package com.zyguo.thread;

import java.util.ArrayList;

public class Main_synchronized {
public static void main(String[] args) {
final User u = new User(1);
int threadNum = 3;
final ThreadGroup tgroup = new ThreadGroup("test-threadgroup");
final ArrayList<Thread> tList = new ArrayList<>();
//定义10个线程
for( int i = 0; i < threadNum; i++ ){
Thread t = new Thread( tgroup, new Runnable() {
@Override
public void run() {
u.addMoney(1);
}
} ,"test-thread-" + i);

tList.add( t );
t.start();
System.out.println("start thread = " + t );
}

//监控线程的活动的子线程数
Thread t = new Thread( new Runnable() {
@Override
public void run() {
int activeCount = tgroup.activeCount();
while ( activeCount > 0 ) {
for (Thread thread : tList) {
System.out.println( thread + ",state=" + thread.getState() );
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
activeCount = tgroup.activeCount();
}
}
});

t.start();

}

}

结果如下

start thread = Thread[test-thread-0,5,test-threadgroup]
start thread = Thread[test-thread-1,5,test-threadgroup]
start thread = Thread[test-thread-2,5,test-threadgroup]
Thread[test-thread-0,5,test-threadgroup],state=TIMED_WAITING
Thread[test-thread-1,5,test-threadgroup],state=BLOCKED
Thread[test-thread-2,5,test-threadgroup],state=BLOCKED
Thread[test-thread-0,5,test-threadgroup],state=TIMED_WAITING
Thread[test-thread-1,5,test-threadgroup],state=BLOCKED
Thread[test-thread-2,5,test-threadgroup],state=BLOCKED
Thread[test-thread-0,5,test-threadgroup],state=TIMED_WAITING
Thread[test-thread-1,5,test-threadgroup],state=BLOCKED
Thread[test-thread-2,5,test-threadgroup],state=BLOCKED
addMoney, currThread=Thread[test-thread-0,5,test-threadgroup]
Thread[test-thread-0,5,],state=TERMINATED
Thread[test-thread-1,5,test-threadgroup],state=BLOCKED
Thread[test-thread-2,5,test-threadgroup],state=TIMED_WAITING
Thread[test-thread-0,5,],state=TERMINATED
Thread[test-thread-1,5,test-threadgroup],state=BLOCKED
Thread[test-thread-2,5,test-threadgroup],state=TIMED_WAITING
Thread[test-thread-0,5,],state=TERMINATED
Thread[test-thread-1,5,test-threadgroup],state=BLOCKED
Thread[test-thread-2,5,test-threadgroup],state=TIMED_WAITING
addMoney, currThread=Thread[test-thread-2,5,test-threadgroup]
Thread[test-thread-0,5,],state=TERMINATED
Thread[test-thread-1,5,test-threadgroup],state=TIMED_WAITING
Thread[test-thread-2,5,],state=TERMINATED
Thread[test-thread-0,5,],state=TERMINATED
Thread[test-thread-1,5,test-threadgroup],state=TIMED_WAITING
Thread[test-thread-2,5,],state=TERMINATED
Thread[test-thread-0,5,],state=TERMINATED
Thread[test-thread-1,5,test-threadgroup],state=TIMED_WAITING
Thread[test-thread-2,5,],state=TERMINATED
addMoney, currThread=Thread[test-thread-1,5,test-threadgroup]