Java多线程2 多个线程之间共享数据
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
1.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
2.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。
(1)对基本类型的数据的封装，这种应用相对很少见。
(2)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
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
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
