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Java 多线程之间共享数据

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0705

这篇文章主要介绍了Java 多线程之间共享数据，围绕Java 多线程之间共享数据展开文章内容线程范围的共享变量、使用Map实现线程范围内数据的共享、ThreadLocal实现线程范围内数据的共享,需要的朋友可以参考一下

1、线程范围的共享变量

多个业务模块针对同一个static变量的操作 要保证在不同线程中 各模块操作的是自身对应的变量对象

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public class ThreadScopeSharaData {
 
    private static  int data = 0 ;
 
    public static void main(String[] args) {
        for(int i = 0 ;i<2 ;i++){
            new Thread(new Runnable(){
 
                @Override
                public void run() {
                    data = new Random().nextInt();
                    System.out.println(Thread.currentThread().getName()+ " put random data:"+data);
                    new A().get() ;
                    new B().get() ;
                }
 
            }).start() ;
        }
 
    }
 
    static class A {
        public int get(){
            System.out.println("A from " + Thread.currentThread().getName()
                    + " get data :" + data);
            return data ;
        }
    }
 
    static class B{
        public int get(){
            System.out.println("B from " + Thread.currentThread().getName()
                    + " get data :" + data);
            return data ;
        }
    }
}

模块A ，B都需要访问static的变量data 在线程0中会随机生成一个data值 假设为10 那么此时模块A和模块B在线程0中得到的data的值为10 ；在线程1中 假设会为data赋值为20 那么在当前线程下

模块A和模块B得到data的值应该为20

看程序执行的结果：

Thread-0 put random data:-2009009251
Thread-1 put random data:-2009009251
A from Thread-0 get data :-2009009251
A from Thread-1 get data :-2009009251
B from Thread-0 get data :-2009009251
B from Thread-1 get data :-2009009251


Thread-0 put random data:-2045829602
Thread-1 put random data:-1842611697
A from Thread-0 get data :-1842611697
A from Thread-1 get data :-1842611697
B from Thread-0 get data :-1842611697
B from Thread-1 get data :-1842611697

会出现两种情况：

  • 1.由于线程执行速度，新的随机值将就的随机值覆盖 data 值一样
  • 2.data 值不一样，但 A、B线程都

2、使用Map实现线程范围内数据的共享

可是将data数据和当前允许的线程绑定在一块，在模块A和模块B去获取数据data的时候 是通过当前所属的线程去取得data的结果就行了。
声明一个Map集合 集合的Key为Thread 存储当前所属线程 Value 保存data的值，

代码如下：

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public class ThreadScopeSharaData {
 
 
    private static Map<Thread, Integer> threadData = new HashMap<>();
 
    public static void main(String[] args) {
 
        for (int i = 0; i < 2; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    int data = new Random().nextInt();
                    System.out.println(Thread.currentThread().getName() + " put random data:" + data);
                    threadData.put(Thread.currentThread(), data);
                    new A().get();
                    new B().get();
 
                }
            }).start();
 
        }
 
    }
 
    static class A {
        public void get() {
            int data = threadData.get(Thread.currentThread());
 
            System.out.println("A from " + Thread.currentThread().getName() + " get data:" + data);
 
        }
    }
 
    static class B {
        public void get() {
            int data = threadData.get(Thread.currentThread());
            System.out.println("B from " + Thread.currentThread().getName() + " get data:" + data);
 
        }
    }
}

Thread-0 put random data:-123490895
Thread-1 put random data:-1060992440
A from Thread-0 get data:-123490895
A from Thread-1 get data:-1060992440
B from Thread-0 get data:-123490895
B from Thread-1 get data:-1060992440

Java 多线程之间共享数据

3、ThreadLocal实现线程范围内数据的共享

(1)订单处理包含一系列操作：减少库存量、增加一条流水台账、修改总账，这几个操作要在同一个事务中完成，通常也即同一个线程中进行处理，如果累加公司应收款的操作失败了，则应该把前面的操作回滚，否则，提交所有操作，这要求这些操作使用相同的数据库连接对象，而这些操作的代码分别位于不同的模块类中。

(2)银行转账包含一系列操作： 把转出帐户的余额减少，把转入帐户的余额增加，这两个操作要在同一个事务中完成，它们必须使用相同的数据库连接对象，转入和转出操作的代码分别是两个不同的帐户对象的方法。

(3)例如Strut2的ActionContext，同一段代码被不同的线程调用运行时，该代码操作的数据是每个线程各自的状态和数据，对于不同的线程来说，getContext方法拿到的对象都不相同，对同一个线程来说，不管调用getContext方法多少次和在哪个模块中getContext方法，拿到的都是同一个。

(4)实验案例：定义一个全局共享的ThreadLocal变量，然后启动多个线程向该ThreadLocal变量中存储一个随机值，接着各个线程调用另外其他多个类的方法，这多个类的方法中读取这个ThreadLocal变量的值，就可以看到多个类在同一个线程中共享同一份数据。

(5)实现对ThreadLocal变量的封装，让外界不要直接操作ThreadLocal变量。

  • 对基本类型的数据的封装，这种应用相对很少见。
  • 对对象类型的数据的封装，比较常见，即让某个类针对不同线程分别创建一个独立的实例对象。
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public class ThreadLocalTest {
 
    private static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
 
    public static void main(String[] args) {
 
        for (int i = 0; i < 2; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    int data = new Random().nextInt();
                    System.out.println(Thread.currentThread().getName() + " put random data:" + data);
                    threadLocal.set(data);
                    new A().get();
                    new B().get();
 
                }
            }).start();
 
        }
 
    }
 
    static class A {
        public void get() {
            int data = threadLocal.get();
 
            System.out.println("A from " + Thread.currentThread().getName() + " get data:" + data);
 
        }
    }
 
    static class B {
        public void get() {
            int data = threadLocal.get();
            System.out.println("B from " + Thread.currentThread().getName() + " get data:" + data);
 
        }
    }
}

Thread-0 put random data:-2015900409
Thread-1 put random data:-645411160
A from Thread-0 get data:-2015900409
A from Thread-1 get data:-645411160
B from Thread-0 get data:-2015900409
B from Thread-1 get data:-645411160

4、优化

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public class ThreadLocalTest {
 
    private static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
 
    //private static ThreadLocal<MyThreadScopeData> myThreadScopeDataThreadLocal = new ThreadLocal<>();
 
 
    public static void main(String[] args) {
 
        for (int i = 0; i < 2; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    int data = new Random().nextInt();
                    System.out.println(Thread.currentThread().getName() + " put random data:" + data);
                    threadLocal.set(data);
 
//                    MyThreadScopeData myThreadScopeData = new MyThreadScopeData();
//                    myThreadScopeData.setName("name" + data);
//                    myThreadScopeData.setAge(data);
//                    myThreadScopeDataThreadLocal.set(myThreadScopeData);
 
                    //获取与当前线程绑定的实例并设置值 
                    MyThreadScopeData.getThreadInstance().setName("name" + data);
                    MyThreadScopeData.getThreadInstance().setAge(data);
                    new A().get();
                    new B().get();
 
                }
            }).start();
 
        }
 
    }
 
    static class A {
        public void get() {
            int data = threadLocal.get();
 
 
//            MyThreadScopeData myData = myThreadScopeDataThreadLocal.get();
//
//
//            System.out.println("A from " + Thread.currentThread().getName()
//                    + " getMyData: " + myData.getName() + "," + myData.getAge());
 
            MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
            System.out.println("A from " + Thread.currentThread().getName()
                    + " getMyData: " + myData.getName() + "," + myData.getAge());
        }
    }
 
    static class B {
        public void get() {
            int data = threadLocal.get();
            //System.out.println("B from " + Thread.currentThread().getName() + " get data:" + data);
 
            MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
            System.out.println("B from " + Thread.currentThread().getName()
                    + " getMyData: " + myData.getName() + "," + myData.getAge());
        }
    }
}
 
//一个绑定当前线程的类
class MyThreadScopeData {
 
    private static ThreadLocal<MyThreadScopeData> map = new ThreadLocal<>();
    private String name;
    private int age;
 
    private MyThreadScopeData() {
    }
 
    //定义一个静态方法，返回各线程自己的实例
    //这里不必用同步，因为每个线程都要创建自己的实例，所以没有线程安全问题。
    public static MyThreadScopeData getThreadInstance() {
        //获取当前线程绑定的实例
        MyThreadScopeData instance = map.get();
        if (instance == null) {
            instance = new MyThreadScopeData();
            map.set(instance);
        }
        return instance;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public int getAge() {
        return age;
    }
 
    public void setAge(int age) {
        this.age = age;
    }
 
 
}

Thread-1 put random data:-1041517189
Thread-0 put random data:-98835751
A from Thread-1 getMyData: name-1041517189,-1041517189
A from Thread-0 getMyData: name-98835751,-98835751
B from Thread-1 getMyData: name-1041517189,-1041517189
B from Thread-0 getMyData: name-98835751,-98835751

5、实例

设计4个线程，其中两个线程每次对j增加1，另外两个线程对j每次减少1，写出程序。

(1)如果每个线程执行的代码相同，可以使用同一个Runnable对象，这个Runnable对象中有那个共享数据，例如，卖票系统就可以这么做。

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public class SellTicket {
    //卖票系统，多个窗口的处理逻辑是相同的
    public static void main(String[] args) {
        Ticket t = new Ticket();
        new Thread(t).start();
        new Thread(t).start();
    }
}
 
/**
 * 将属性和处理逻辑，封装在一个类中
 *
 * @author yang
 */
class Ticket implements Runnable {
 
    private int ticket = 10;
 
    public synchronized void run() {
        while (ticket > 0) {
            ticket--;
            System.out.println("当前票数为：" + ticket);
        }
    }
}

(2)如果每个线程执行的代码不同，这时候需要用不同的Runnable对象，例如，设计2个线程。一个线程对j增加1，另外一个线程对j减1，银行存取款系统。

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public class MultiThreadShareData {
    private int j;
    public static void main(String[] args) {
        MultiThreadShareData multiThreadShareData = new MultiThreadShareData();
        for(int i=0;i<2;i++){
            new Thread(multiThreadShareData.new ShareData1()).start();//增加
            new Thread(multiThreadShareData.new ShareData2()).start();//减少
        }
    }
    //自增
    private synchronized void Inc(){
        j++;
        System.out.println(Thread.currentThread().getName()+" inc "+j);
    }
    //自减
    private synchronized void Dec(){
        j--;
        System.out.println(Thread.currentThread().getName()+" dec "+j);
    }
 
    class ShareData1 implements Runnable {
        public void run() {
            for(int i=0;i<5;i++){
                Inc();
            }
        }
    }
    class ShareData2 implements Runnable {
        public void run() {
            for(int i=0;i<5;i++){
                Dec();
            }
        }
    }
}

Thread-0 inc 1
Thread-0 inc 2
Thread-0 inc 3
Thread-0 inc 4
Thread-0 inc 5
Thread-1 dec 4
Thread-1 dec 3
Thread-2 inc 4
Thread-2 inc 5
Thread-2 inc 6
Thread-2 inc 7
Thread-2 inc 8
Thread-1 dec 7
Thread-1 dec 6
Thread-1 dec 5
Thread-3 dec 4
Thread-3 dec 3
Thread-3 dec 2
Thread-3 dec 1
Thread-3 dec 0

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原文链接：https://juejin.cn/post/7016934881129136159