如果多个线程使用同一个数据，那么如何保证线程范围内的数据共享。
我们可以使用一个map来存储当前线程，以及其数据如下：
package andy.thread.traditional.test;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
/**
* @author Zhang,Tianyou
* @version 2014年11月8日 下午2:12:44
*/
// 线程范围内的共享数据
public class ThreadScopeSharedData {
private static int data = 0;
//ThreadLocal 提供了线程范围内的数据共享 只能存贮一个数据
private static Map<Thread, Integer> threadData = new HashMap<Thread, Integer>();
//private static ThreadLocal<Integer> x = new ThreadLocal<Integer>();
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()
+ " has put data :" + data);
threadData.put(Thread.currentThread(), data);
//x.set(data);
new A().get();
new B().get();
}
}).start();
}
}
static class A {
public void get() {
int data = threadData.get(Thread.currentThread());
//int data = x.get();
System.out.println("A from " + Thread.currentThread().getName()
+ " get data :" + data);
}
}
static class B {
public void get() {
int data = threadData.get(Thread.currentThread());
//int data = x.get();
System.out.println("B from " + Thread.currentThread().getName()
+ " get data :" + data);
}
}
}
运行效果如下：
Thread-1 has put data :-591164568 Thread-0 has put data :2032497041 A from Thread-0 get data :2032497041 A from Thread-1 get data :-591164568 B from Thread-0 get data :2032497041 B from Thread-1 get data :-591164568
但JDK也为我们提供了另一种实现方式：ThreadLocal类，它能够实现各自线程局部变量，并且独立于变量的初始化副本。
-
public class ThreadLocal<T>
- extends Object
该类提供了线程局部 (thread-local) 变量。这些变量不同于它们的普通对应物，因为访问某个变量（通过其 get 或 set 方法）的每个线程都有自己的局部变量，它独立于变量的初始化副本。ThreadLocal 实例通常是类中的 private static 字段，它们希望将状态与某一个线程（例如，用户 ID 或事务 ID）相关联。
例如，以下类生成对每个线程唯一的局部标识符。线程 ID 是在第一次调用 UniqueThreadIdGenerator.getCurrentThreadId() 时分配的，在后续调用中不会更改。
import java.util.concurrent.atomic.AtomicInteger;
public class UniqueThreadIdGenerator {
private static final AtomicInteger uniqueId = new AtomicInteger(0);
private static final ThreadLocal < Integer > uniqueNum =
new ThreadLocal < Integer > () {
@Override protected Integer initialValue() {
return uniqueId.getAndIncrement();
}
};
public static int getCurrentThreadId() {
return uniqueId.get();
}
} // UniqueThreadIdGenerator
每个线程都保持对其线程局部变量副本的隐式引用，只要线程是活动的并且 ThreadLocal 实例是可访问的；在线程消失之后，其线程局部实例的所有副本都会被垃圾回收（除非存在对这些副本的其他引用）。
上面代码的更优雅的实现如下：
package andy.thread.traditional.test;
import java.util.Random;
/**
* @author Zhang,Tianyou
* @version 2014年11月8日 下午2:26:24
*/
public class ThreadLocalTest {
private static int data = 0;
// ThreadLocal 提供了线程范围内的数据共享
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()
+ " has put data :" + data);
MyThreadScopeData.getThreadScopeInstance().setName(
"name = " + data);
MyThreadScopeData.getThreadScopeInstance().setAge(data);
new A().get();
new B().get();
}
}).start();
}
}
static class A {
public void get() {
MyThreadScopeData myThreadScopeData = MyThreadScopeData
.getThreadScopeInstance();
System.out.println("A from " + Thread.currentThread().getName()
+ " Name :" + myThreadScopeData.getName() + "age = "
+ myThreadScopeData.getAge());
}
}
static class B {
public void get() {
MyThreadScopeData myThreadScopeData = MyThreadScopeData
.getThreadScopeInstance();
System.out.println("B from " + Thread.currentThread().getName()
+ " Name :" + myThreadScopeData.getName() + "age = "
+ myThreadScopeData.getAge());
}
}
}
class MyThreadScopeData {
private MyThreadScopeData() {
}
private static ThreadLocal<MyThreadScopeData> map = new ThreadLocal<MyThreadScopeData>();
// 此处不需要synchronized 应该每个线程操作各自的数据
public static MyThreadScopeData getThreadScopeInstance() {
// 返回此线程局部变量的当前线程副本中的值。
MyThreadScopeData instance = map.get();
if (instance == null) {
instance = new MyThreadScopeData();
// 将此线程局部变量的当前线程副本中的值设置为指定值。
map.set(instance);
}
return instance;
}
private String name;
private int age;
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;
}
}
其运行效果如下：
Thread-1 has put data :1322483490 A from Thread-1 Name :name = 1322483490age = 1322483490 B from Thread-1 Name :name = 1322483490age = 1322483490 Thread-0 has put data :1149801771 A from Thread-0 Name :name = 1149801771age = 1149801771 B from Thread-0 Name :name = 1149801771age = 1149801771
时间： 2024-10-25 00:12:41