再看ThreadLocal

最新推荐文章于 2025-05-13 16:09:15 发布

原创最新推荐文章于 2025-05-13 16:09:15 发布 · 550 阅读

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本文探讨了如何在Java线程中共享变量，并通过ThreadLocal解决线程间变量冲突问题，同时展示了ThreadLocal在实现线程内单例模式的应用。

1.线程能共享数据

线程内部要用到线程外的一个变量

package com.lgc;

import java.util.HashMap;
import java.util.Map;
import java.util.Random;

public class ThreadScopeShareData {

	private static int data = 0;
	public static void main(String[] args) {
		for(int i=0;i<3;i++){
			new Thread(new Runnable(){
				@Override
				public void run() {
					data = new Random().nextInt();
					System.out.println(Thread.currentThread().getName() 
							+ " has put data :" + data);
					new A().get();
					new B().get();
				}
			}).start();
		}
	}
	
	static class A{
		public void get(){
			System.out.println("A from " + Thread.currentThread().getName() 
					+ " get data :" + data);
		}
	}
	
	static class B{
		public void get(){
			System.out.println("B from " + Thread.currentThread().getName() 
					+ " get data :" + data);
		}		
	}
}
out：
Thread-0 has put data :1708994183
Thread-1 has put data :-112088840
A from Thread-0 get data :-112088840
A from Thread-1 get data :-112088840
B from Thread-0 get data :-112088840
B from Thread-1 get data :-112088840

线程0放了一个 data 1708994183 ，当他后面要用data 取出来的却是112088840 ，在一个类里当然可以设个局部变量用来缓存。

要想任何属于这个线程的类都能取到正确的data怎么解决？

线程内共享变量

package com.lgc;

import java.util.HashMap;
import java.util.Map;
import java.util.Random;

public class ThreadScopeShareData {

	private static int data = 0;
	private static Map<Thread, Integer> threadData = new HashMap<Thread, 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);
					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);
		}		
	}
}

out:

Thread-0 has put data :1156068854
Thread-1 has put data :-347154499
A from Thread-0 get data :1156068854
A from Thread-1 get data :-347154499
B from Thread-0 get data :1156068854

B from Thread-1 get data :-347154499

JDK提供的ThreadLocal就相当于这里的threadData，这就是ThreadLocal解决的问题。

ThreadLocal实现上述问题代码

public class ThreadLocalTest {

	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);
					x.set(data);
					new A().get();
					new B().get();
				}
			}).start();
		}
	}
	
	static class A{
		public void get(){
			int data = x.get();
			System.out.println("A from " + Thread.currentThread().getName() 
					+ " get data :" + data);
		}
	}
	
	static class B{
		public void get(){
			int data = x.get();			
			System.out.println("B from " + Thread.currentThread().getName() 
					+ " get data :" + data);
		}		
	}
}

2. ThreadLocal的另一个应用(线程内单例模式)

class MyThreadScopeData{
	private MyThreadScopeData(){}
	public static  MyThreadScopeData getThreadInstance(){
		MyThreadScopeData instance = map.get();
		if(instance == null){
			instance = new MyThreadScopeData();
			map.set(instance);
		}
		return instance;
	}
	private static ThreadLocal<MyThreadScopeData> map = new ThreadLocal<MyThreadScopeData>();
	
}