Java 多线程之间共享数据
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
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:
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
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*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。
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
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。
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。
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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