实战java高并发程序设计第二章

摘要：线程的基本状态线程的基本操作与内存模型线程组守护线程线程优先级线程安全与隐蔽错误线程的基本状态线程的生命周期线程的基本操作新建线程终止线程立即终止线程所有活动方法在结束线程时会直接终止线程并立即释放这个线程所持有的锁可能引起数据不一致强烈建

 1.线程的基本状态 
 2.线程的基本操作 
 3. volatile与java内存模型 
 4.线程组 
 5.守护线程(Daemon) 
 6.线程优先级 
 7.线程安全与synchronized 
 8.隐蔽错误 

线程的生命周期

1.新建线程

    Thread tl=new Thread(){
        @override
        public void run(){
            System.out.println("Hel1o,I am t1");
        };
        t1.start();
    t1.start();

2.终止线程

Thread.stop()    //立即终止线程所有活动

stop()方法在结束线程时,会直接终止线程,并立即释放这个线程所持有的锁,可能引起数据不一致,强烈建议不使用!!

    //正确停止线程的方法
    public static class ChangeObjectThread extends Thread {
        volatile boolean stopme = false;     //标记位,记录是否要停止线程
        
        public void stopMe(){
            stopme = true;
        }
        @Override
        public void run() {
            while (true) {
                if (stopme){                //在合适的地方停止线程,避免数据不一致
                    System.out.println("exit by stop me");
                    break;
                }
                synchronized (u) {
                    int v = (int) (System.currentTimeMillis() / 1000);
                    u.setId(v);
                    //Oh, do sth. else
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    u.setName(String.valueOf(v));
                }
                Thread.yield();
            }
        }
    }

3.线程中断

public void Thread.interrupt()        //中断线程,仅发送通知,设置标记位,等待合适时机线程中断
public boolean Thread.isInterrupted()    //判断是否被中断,判断标记位
public static boolean Thread.interrupted()//判断是否被中断，并清除当前中断状态

Thread tl=new Thread(）{
    @Override
    public void run(){
        while(true){
            if（Thread.currentThread().isInterrupted()){    //需要设立中断处理方法,否则无法响应中断
                System.out.println("Interruted!");
                break；
            ）
            Thread.yield()；
}

public class InterruputSleepThread {
    public static void main(String[] args) throws InterruptedException {
        Thread t1=new Thread(){
            @Override
            public void run(){
                while(true){
                    if(Thread.currentThread().isInterrupted()){
                        System.out.println("Interruted!");
                        break;
                    }
                    try {
                        Thread.sleep(2000);            //由于中断而抛出异常会清除中断标记
                    } catch (InterruptedException e) {
                        System.out.println("Interruted When Sleep");    //sleep被中断后,重置标记位继续运行,保证数据一致性和完整性
                        //设置中断状态
                        Thread.currentThread().interrupt();
                    }
                    Thread.yield();
                }
            }
        };
        t1.start();
        Thread.sleep(2000);
        t1.interrupt();
    }
}

4.等待(wait)和通知(notify)

public final void wait() throws InterruptedException;
public final native void notify();

object.wait()和object.notify()方法必须包含在synchronized关键字中,因为要获取对象的监视器(monitor对象)

public class SimpleWN {
    final static Object object = new Object();
    public static class T1 extends Thread{
        public void run()
        {
            synchronized (object) {
                System.out.println(System.currentTimeMillis()+":T1 start! ");
                try {
                    System.out.println(System.currentTimeMillis()+":T1 wait for object ");
                    object.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(System.currentTimeMillis()+":T1 end!");
            }
        }
    }
    public static class T2 extends Thread{
        public void run()
        {
            synchronized (object) {
                System.out.println(System.currentTimeMillis()+":T2 start! notify one thread");
                object.notify();
                System.out.println(System.currentTimeMillis()+":T2 end!");
                try {
                    Thread.sleep(2000);        //此处sleep()2秒,T1此时仍然需要等待2秒才会继续执行
                } catch (InterruptedException e) {
                }
            }
        }
    }
    public static void main(String[] args) {
        Thread t1 = new T1() ;
        Thread t2 = new T2() ;
        t1.start();
        t2.start();
        //输出:
        //1565249714094:T1 start! 
        //1565249714094:T1 wait for object 
        //1565249714094:T2 start! notify one thread
        //1565249714094:T2 end!
        //1565249716094:T1 end!
    }
} 

    wait()和sleep()的区别:
1.wait必须在同步条件内使用,sleep无此要求
2.wait释放锁,sleep不会释放
3.wait为Object内的实例方法,sleep为Thread的静态方法
4.wait需要调用对象的notify方法唤醒,sleep知道时间结束或者被interrupt打断

5.挂起(suspend)和继续执行(resume)

废弃方法,不作介绍!!有需求可使用wait和notify

6.等待线程结束(join)和谦让yield

public final void join() throws InterruptedException    //一直阻塞线程直至结束
public final synchronized vold join(long millis)throws InterruptedException    //给出最大等待时间

join的机制实际为在方法调用线程循环wait,如下join()核心代码:

while(isAlive()){
    wait(0);
}

public static native void yield();

yield()方法表示当前线程让出cpu计算资源,但还是会参与cpu资源的争夺,一般用于防止低优先级线程占用太多cpu资源

volatile关键字可以保证一定的原子性,但是不能替代锁,不能保证100%线程安全
volatile可以保证可见性和有序性,充当内存栅栏,禁止指令重拍

  当一个系统中线程较多且功能分配比较明确是可以使用线程组
  

public class ThreadGroupName implements Runnable {
    public static void main(String[] args) {
        ThreadGroup tg = new ThreadGroup("PrintGroup");
        Thread t1 = new Thread(tg, new ThreadGroupName(), "T1");
        Thread t2 = new Thread(tg, new ThreadGroupName(), "T2");
        t1.start();
        t2.start();
        System.out.println(tg.activeCount());    //2
        tg.list();        //java.lang.ThreadGroup[name=PrintGroup,maxpri=10]
                          //    Thread[T1,5,PrintGroup]
                          //    Thread[T2,5,PrintGroup]
    }
    @Override
    public void run() {
        String groupAndName = Thread.currentThread().getThreadGroup().getName()
                + "-" + Thread.currentThread().getName();
        while (true) {
            System.out.println("I am " + groupAndName);
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

activeCount()获取线程组中的活动线程数
list()则打印线程组中的所有线程信息

守护线程在后台执行,如垃圾回收线程,与之对应的为用户线程,当守护线程要守护的对象不存在了,则守护线程自动退出
当一个java应用中只有守护线程时,java虚拟机自动退出

public class DaemonDemo {
    public static class DaemonT extends Thread{
        public void run(){
            while(true){
                System.out.println("I am alive");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    public static void main(String[] args) throws InterruptedException {
        Thread t=new DaemonT();
        t.setDaemon(true);    //此方法必须在start方法前
        t.start();
        
        Thread.sleep(2000);
    }
}

上述代码中 t被设置为守护线程,当主线程退出时,守护线程也会退出;若未设置为守护线程,主线程退出后,t线程将依旧打印.

线程可以设置优先级,优先级高的线程获取到cpu资源的概率会更大(但不是一定会比低优先级的线程先获取到cpu资源)

public class PriorityDemo {
    public static class HightPriority extends Thread{
        static int count=0;
        public void run(){
            while(true){
                synchronized(PriorityDemo.class){       //此处锁住该类是为了让产生一次资源竞争,使优先级差异表现更为明显
                    count++;
                    if(count>10000000){
                        System.out.println("HightPriority is complete");
                        break;
                    }
                }
            }
        }
    }
    public static class LowPriority extends Thread{
        static int count=0;
        public void run(){
            while(true){
                synchronized(PriorityDemo.class){
                    count++;
                    if(count>10000000){
                        System.out.println("LowPriority is complete");
                        break;
                    }
                }
            }
        }
    }
    /**
     * HightPriority先完成的次数多，但是 不保证
     * @param args
     * @throws InterruptedException
     */
    public static void main(String[] args) throws InterruptedException {
        Thread high=new HightPriority();
        LowPriority low=new LowPriority();
        high.setPriority(Thread.MAX_PRIORITY);    //此方法可以设置优先级 0-10 数字
        low.setPriority(Thread.MIN_PRIORITY);
        low.start();
        high.start();
    }
}

试验多次HightPriority总是能比LowPriority快,但是不保证一定是这样.

synchronized为重量级同步锁,实现线程间的同步,sychronize锁住的对象、方法和代码块,每一次只能有一个线程进行访问,从而保证了线程的安全

    用法:
    
指定锁对象: synchronized(object){.....}
指定实例方法 : public  synchronized void println()
指定静态方法: public static synchronized void println()

public class BadAccountingSync2 implements Runnable{
    static int i=0;
    public static synchronized void increase(){        //此处如果不适用static则为线程不安全,最终i值必定小于20000
        i++;
    }
    @Override
    public void run() {
        for(int j=0;j<10000;j++){
            increase();
        }
    }
    public static void main(String[] args) throws InterruptedException {
        Thread t1=new Thread(new BadAccountingSync2());
        Thread t2=new Thread(new BadAccountingSync2());
        t1.start();t2.start();
        t1.join();t2.join();
        System.out.println(i);     //输出20000
    }
}

数值溢出

int v1=1073741827;
int v2=1431655768；
System.out,println("vl="+v1)；    //1073741827
System.out.println("v2="+v2)；    //1431655768
int ave=(vl+v2)/2;
System.out,println("ave="+ave);    //-894784850  数值溢出,两int数值相加超过int最大值时溢出

并发不安全的arraylist

public class ArrayListMultiThread {
    static ArrayList al = new ArrayList(10);
    public static class AddThread implements Runnable {
        @Override
        public void run() {
            for (int i = 0; i < 1000000; i++) {
                al.add(i);
            }
        }
    }
    public static void main(String[] args) throws InterruptedException {
        Thread t1=new Thread(new AddThread());
        Thread t2=new Thread(new AddThread());
        t1.start();
        t2.start();
        t1.join();t2.join();
        System.out.println(al.size());
    }
}

以上程序可能会有三个结果
1.正常结束,打印值为2000000    (运气好)
2.正常结束,打印值小于2000000    (一般情况下会出现这种并发不安全的现象)
3.抛出异常,arrayindexoutofbound(扩容过程中内部一致性遭破坏,导致数组越界)

如何避免此问题?
使用线程安全的vector代替arraylist

并发不安全的hashmap

public class JDK8HashMapMultiThread {
    static Map map = new HashMap();
    static int nullNum = 0;     //记录空值数
    public static class AddThread implements Runnable {
        int start = 0;
        public AddThread(int start) {
            this.start = start;
        }
        @Override
        public void run() {
            for (int i = start; i < 10000; i += 2) {
                map.put(Integer.toString(i), Integer.toString(i));
            }
        }
    }
    public static void test() throws Exception {
        Thread t1 = new Thread(new JDK8HashMapMultiThread.AddThread(0));
        Thread t2 = new Thread(new JDK8HashMapMultiThread.AddThread(1));
        t1.start();
        t2.start();
        t1.join();
        t2.join();
    }
    public static void main(String[] args) throws Exception {
        test();
        for (int i = 0; i < 10000; i++) {
            if (map.get("" + i) == null) {
                nullNum += 1;
            }
        }
        System.out.println(10000 - nullNum);  //hashmap中存有的entry数
        System.out.println(map.size());     //hashmap中size属性的值
    }
}

打印值为: 6573
         9715
打印结果不一定,但可以看出线程一定不安全,原因在于:
1.put时,两个entry的key hash值一致,同时进入插入代码,导致只有一个值插入成功
2.插入成功后size数值 进行size++,非原子操作,导致并发不安全

注意:
另网上有其他博文说会产生resize闭链问题,此问题只存在jdk1.8版本以前,1.8版本已优化过hashmap,故不存在闭链问题
想要避免并发问题可以使用concurrenthashmap

错误的加锁

public class BadLockOnInteger implements Runnable{
    public static Integer i=0;        //integer为不变类型,对其加锁无效
    static BadLockOnInteger instance=new BadLockOnInteger();
    @Override
    public void run() {
        for(int j=0;j<1000000;j++){
            synchronized(i){
//            synchronized(instance){    //改为对instance加锁即线程安全了
                i++;
            }
        }
    }
    public static void main(String[] args) throws InterruptedException {
        Thread t1=new Thread(instance);
        Thread t2=new Thread(instance);
        t1.start();t2.start();
        t1.join();t2.join();
        System.out.println(i);        //打印值小于2000000
        
    }
}

注意:
对于不变对象如基本类型 Integer String 等加锁,相当于每次都锁不同的对象,从而导致并发问题产生.