Java并发编程实战：第1章 简介

下载资源 jar；https://repo1.maven.org/maven2/org/lucee/jcip-annotations/1.0.0/jcip-annotations-1.0.0.jar

添加 jar 到程序中

1. 线程不安全

多个线程中的操作执行顺序是不可预测的

非线程安全的序列生成器

@NotThreadSafe
class UnsafeSequence {
    private int value;

    public UnsafeSequence(int value) {
        this.value = value;
    }

    public int next() {
        return this.value++;
    }
}

创建3个线程进行测试：

public class UnsafeSequenceDemo {
    public static void main(String[] args) {
        UnsafeSequence unsafeSequence = new UnsafeSequence(100);

        MyThread thread = new MyThread(unsafeSequence);
        Thread myThread1 = new Thread(thread, "线程1");
        Thread myThread2 = new Thread(thread, "线程2");
        Thread myThread3 = new Thread(thread, "线程3");

        myThread1.start();
        myThread2.start();
        myThread3.start();
    }
}


class MyThread implements Runnable {

    private UnsafeSequence unsafeSequence;

    public MyThread(UnsafeSequence unsafeSequence) {
        this.unsafeSequence = unsafeSequence;
    }

    @Override
    public void run() {
        // print
        System.out.println(Thread.currentThread().getName() + "------------------ " + unsafeSequence.next() + " ------------------");
    }
}

output:

线程2------------------ 100 ------------------
线程1------------------ 100 ------------------
线程3------------------ 101 ------------------

原因分析，可能情况：

1. 线程2 启动，value=100 进行输出 100
2. 线程1 启动，value=100 进行输出 100
3. 线程2 value++，value=101
4. 线程3 启动，value=101 进行输出 101
5. 线程1、线程3 也进行 value++，value=103

2. 线程安全

线程安全的数值序列生成器

@ThreadSafe
class Sequence {
    @GuardedBy("this")
    private int value;

    public Sequence(int value) {
        this.value = value;
    }

    public synchronized int next() {
        return this.value++;
    }
}

创建3个线程进行测试：

public class SequenceDemo {
    public static void main(String[] args) {
        Sequence sequence = new Sequence(100);

        MeThread thread = new MeThread(sequence);
        Thread meThread1 = new Thread(thread, "线程1");
        Thread meThread2 = new Thread(thread, "线程2");
        Thread meThread3 = new Thread(thread, "线程3");

        meThread1.start();
        meThread2.start();
        meThread3.start();
    }
}

class MeThread implements Runnable {

    private Sequence sequence;

    public MeThread(Sequence sequence) {
        this.sequence = sequence;
    }

    @Override
    public void run() {
        // print
        System.out.println(Thread.currentThread().getName() + "------------------ " + sequence.next() + " ------------------");
    }
}

output：

线程1------------------ 100 ------------------
线程2------------------ 101 ------------------
线程3------------------ 102 ------------------