多线程与并发-----ThreadLocal类及应用技巧

- 日理万妓 2021-09-15 10:04 286阅读 0赞

多个模块在同一个线程中运行时要共享同一份数据，实现线程范围内的数据共享可以用Map。

**JDK1.5提供了ThreadLocal类来方便实现线程范围内的数据共享，它的作用就相当于Map。**

**每个线程调用全局ThreadLocal对象的set方法，就相当于往其内部的map集合中增加一条记录，key就是各自的线程，value就是各自的set方法传进去的值。**

在线程结束时可以调用ThreadLocal.clear()方法用来更快释放内存，也可以不调用，因为线程结束后也可以自动释放相关的ThreadLocal变量。

>     import java.util.HashMap;
>     import java.util.Map;
>     import java.util.Random;
>     
>     public class ThreadLocalTest {
>     	
>     	private static ThreadLocal<Integer> threadLocal=new ThreadLocal<Integer>();
>     	public static void main(String[] args) {
>     		//创建两个线程
>     		for(int i=0;i<2;i++){
>     			new Thread(new Runnable() {
>     				
>     				@Override
>     				public void run() {
>     					//现在当前线程中修改一下数据,给出修改信息
>     					int data=new Random().nextInt();
>     					//Thread.currentThread()获取当前线程。
>     					System.out.println(Thread.currentThread().getName()+" has put data:"+data);
>     					//将线程信息和对应数据存储起来
>     					threadLocal.set(data);
>     					//使用两个不同的模块操作这个数据，看结果
>     					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);
>     		}
>     	}
>     	
>     }
> 
> 运行结果为：

> > > Thread-0 has put data:-1595352605  
> > > Thread-1 has put data:-765849300  
> > > A fromThread-0 get data:-1595352605  
> > > A fromThread-1 get data:-765849300  
> > > B fromThread-0 get data:-1595352605  
> > > B fromThread-1 get data:-765849300

**一个****ThreadLocal****对象只能记录一个线程内部的一个共享变量，需要记录多个共享数据，可以创建多个****ThreadLocal****对象，或者将这些数据进行封装，将封装后的数据对象存入****ThreadLocal****对象中。**

**   运行结果为：**

> > > Thread-0 has put data:1785375164  
> > > Thread-1 has put data:1981597103  
> > > A fromThread-0 get data:1785375164  
> > > A fromThread-1 get data:1981597103  
> > > A fromThread-0 getMyData：name:1785375164,1785375164  
> > > A fromThread-1 getMyData：name:1981597103,1981597103  
> > > B fromThread-0 get data:1785375164  
> > > B fromThread-1 get data:1981597103  
> > > B fromThread-0 getMyData：name:1785375164,1785375164  
> > > 
> > > 
> > > B fromThread-1 getMyData：name:1981597103,1981597103

#### **将上述代码进行优化：** ####

**        将数据对象封装成单例，同时提供线程范围内的共享数据的设置和获取方法，提供已经封装好了的线程范围内的对象实例，使用时只需获取实例对象即可实现数据的线程范围内的共享，因为该对象已经是当前线程范围内的对象了。**

**         演示代码如下： **

>     import java.util.HashMap;
>     import java.util.Map;
>     import java.util.Random;
>     
>     public class ThreadLocalTest {
>     	
>     	private static ThreadLocal<Integer> threadLocal=new ThreadLocal<Integer>();
>     	
>     	private static ThreadLocal<MythreadScopeData> mythreadScopeData=new ThreadLocal<MythreadScopeData>();
>     	
>     	public static void main(String[] args) {
>     		//创建两个线程
>     		for(int i=0;i<2;i++){
>     			new Thread(new Runnable() {
>     				
>     				@Override
>     				public void run() {
>     					//现在当前线程中修改一下数据,给出修改信息
>     					int data=new Random().nextInt();
>     					//Thread.currentThread()获取当前线程。
>     					System.out.println(Thread.currentThread().getName()+" has put data:"+data);
>     					//将线程信息和对应数据存储起来
>     					threadLocal.set(data);
>     					
>     					/*
>     					MythreadScopeData mData=new MythreadScopeData();
>     					mData.setName("name:"+data);
>     					mData.setAge(data);
>     					mythreadScopeData.set(mData);
>     					*/
>     					MythreadScopeData.getThreadInstance().setName("name:"+data);
>     					MythreadScopeData.getThreadInstance().setAge(data);
>     					
>     					//使用两个不同的模块操作这个数据，看结果
>     					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);
>     			
>     			/*MythreadScopeData mData=mythreadScopeData.get();	
>     			System.out.println("A from"+Thread.currentThread().getName()
>     					+" getMyData："+mData.getName()+","+mData.getAge());*/
>     			
>     			MythreadScopeData mData=MythreadScopeData.getThreadInstance();
>     			System.out.println("A from "+Thread.currentThread().getName()
>     					+" getMyData："+mData.getName()+","+mData.getAge());
>     		}
>     	}
>     	
>     	static class B{
>     		public void get(){
>     			int data=threadLocal.get();
>     			System.out.println("B from "+Thread.currentThread().getName()+" get data:"+data);
>     			
>     			/*MythreadScopeData mData=mythreadScopeData.get();	
>     			System.out.println("B from"+Thread.currentThread().getName()
>     					+" getMyData："+mData.getName()+","+mData.getAge());*/
>     			
>     			MythreadScopeData mData=MythreadScopeData.getThreadInstance();
>     			System.out.println("B from "+Thread.currentThread().getName()
>     					+" getMyData："+mData.getName()+","+mData.getAge());
>     		}
>     	}
>     	
>     }
>     
>     class MythreadScopeData{
>     	private MythreadScopeData(){}
>     	//创建单例模式
>     	/*
>     	//方法一
>     	public static MythreadScopeData getInstance(){
>     		return instance;
>     	}
>     	private static MythreadScopeData instance=new MythreadScopeData();
>     	
>     	//方法二
>     	public static synchronized MythreadScopeData getInstance(){
>     		if(instance==null){
>     			instance =new MythreadScopeData();
>     		}
>     		return instance;
>     	}
>     	private static MythreadScopeData instance=null;
>     	*/
>     	
>     	//仿照单例模式，建立单例线程
>     	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>();
>     	
>     	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;
>     	}
>     	
>     }

> 运行结果为：

> > > Thread-1 has put data:-594509613  
> > > Thread-0 has put data:-395209683  
> > > A from Thread-1 get data:-594509613  
> > > A from Thread-0 get data:-395209683  
> > > A from Thread-1 getMyData：name:-594509613,-594509613  
> > > A from Thread-0 getMyData：name:-395209683,-395209683  
> > > B from Thread-1 get data:-594509613  
> > > B from Thread-0 get data:-395209683  
> > > B from Thread-1 getMyData：name:-594509613,-594509613  
> > > B from Thread-0 getMyData：name:-395209683,-395209683  
> > >

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