ThreadLocal使用学习

ThreadLocal字面上的意思是局部线程变量，每个线程通过ThreadLocal的get和set方法来访问和修改线程自己独有的变量。简单地说，ThreadLocal的作用就是为每一个线程提供了一个独立的变量副本，每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。

ThreadLocal的基本使用

ThreadLocal是一个泛型类，在创建的时候需要指定变量的类型：

1 private static ThreadLocal<String> threadLocal = new ThreadLocal<>();

 

ThreadLocal提供了set方法来设置变量的值，get方法获取变量的值，remove方法移除变量：

1234567891011 public class ThreadLocalTest { private static ThreadLocal<String> threadLocal = new ThreadLocal<>(); public static void main(String[] args) throws InterruptedException { threadLocal.set("mrbird"); System.out.println(threadLocal.get()); threadLocal.remove(); System.out.println(threadLocal.get()); }}

 

程序输出如下：

我们也可以给ThreadLocal设置初始值，设置初始值有两种方式：

重写initialValue方法： 1234567891011 public class ThreadLocalTest { private static ThreadLocal<String> threadLocal = new ThreadLocal<String>(){ @Override protected String initialValue() { return "初始值"; } }; public static void main(String[] args) throws InterruptedException { System.out.println(threadLocal.get()); // 初始值 }} 使用ThreadLocal的withInitial方法： 123456 public class ThreadLocalTest { private static ThreadLocal<String> threadLocal = ThreadLocal.withInitial(() -> "初始值"); public static void main(String[] args) throws InterruptedException { System.out.println(threadLocal.get()); // 初始值 }}

值得注意的是remove无法移除初始值：

1234567 public class ThreadLocalTest { private static ThreadLocal<String> threadLocal = ThreadLocal.withInitial(() -> "初始值"); public static void main(String[] args) throws InterruptedException { threadLocal.remove(); System.out.println(threadLocal.get()); // 初始值 }}

 

演示多线程间独立

在多个线程中使用ThreadLocal：

12345678910111213141516171819202122232425262728293031323334 public class ThreadLocalTest2 { private static ThreadLocal<String> threadLocal = new ThreadLocal<>(); private static Random random = new Random(System.currentTimeMillis()); public static void main(String[] args) throws InterruptedException { Thread thread1 = new Thread(() -> { threadLocal.set("thread t1"); try { TimeUnit.MICROSECONDS.sleep(random.nextInt(1000)); System.out.println(Thread.currentThread().getName() + " " + threadLocal.get()); } catch (InterruptedException e) { e.printStackTrace(); } }, "thread1"); Thread thread2 = new Thread(() -> { threadLocal.set("thread t2"); try { TimeUnit.MICROSECONDS.sleep(random.nextInt(1000)); System.out.println(Thread.currentThread().getName() + " " + threadLocal.get()); } catch (InterruptedException e) { e.printStackTrace(); } }, "thread2"); thread1.start(); thread2.start(); thread1.join(); thread2.join(); System.out.println(Thread.currentThread().getName() + " " + threadLocal.get()); }}

 

程序输出如下：

结果证明了ThreadLocal在每个线程间是相互独立的，threadLocal在thread1、thread2和main线程间都有一份独立拷贝。

ThreadLocal基本原理

在ThreadLocal类中有一个静态内部类ThreadLocalMap(概念上类似于Map)，用键值对的形式存储每一个线程的变量副本，ThreadLocalMap中元素的key为当前ThreadLocal对象，而value对应线程的变量副本。

我们使用Map来代替ThreadLocalMap，创建一个简易的类ThreadLocal实现：

123456789101112131415161718192021222324252627 public class MyThreadLocal<T> { private final Map<Thread, T> threadLocalMap = new HashMap<>(); public void set(T t) { synchronized (this) { Thread key = Thread.currentThread(); threadLocalMap.put(key, t); } } public T get() { synchronized (this) { Thread key = Thread.currentThread(); T t = threadLocalMap.get(key); if (t == null) { return initalValue(); } else { return t; } } } public T initalValue() { return null; }}

 

使用方式和之前的例子一致：

123456789101112131415161718192021222324252627282930313233343536373839 public class ThreadLocalTest3 { private static MyThreadLocal<String> threadLocal = new MyThreadLocal<String>() { @Override public String initalValue() { return "initalValue"; } }; private static Random random = new Random(System.currentTimeMillis()); public static void main(String[] args) throws InterruptedException { Thread thread1 = new Thread(() -> { threadLocal.set("thread t1"); try { TimeUnit.MICROSECONDS.sleep(random.nextInt(1000)); System.out.println(Thread.currentThread().getName() + " " + threadLocal.get()); } catch (InterruptedException e) { e.printStackTrace(); } }, "thread1"); Thread thread2 = new Thread(() -> { threadLocal.set("thread t2"); try { TimeUnit.MICROSECONDS.sleep(random.nextInt(1000)); System.out.println(Thread.currentThread().getName() + " " + threadLocal.get()); } catch (InterruptedException e) { e.printStackTrace(); } }, "thread2"); thread1.start(); thread2.start(); thread1.join(); thread2.join(); System.out.println(Thread.currentThread().getName() + " " + threadLocal.get()); }}

 

程序输出如下：

使用建议

将ThreadLocal变量指定为private static；

使用完毕后显式地调用remove方法移除。

转载于:https://www.cnblogs.com/7788IT/p/11426479.html

相关资源：JAVA上百实例源码以及开源项目