ThreadLocal
变量值得共享可以使用public static变量的形式,所有的线程都使用同一个public static变量。如果想实现每一个线程都有自己的共享变量该如何解决呢?JDK中提供的ThreadLocal正是解决这样的问题。
ThreadLocal主要解决的就是每个线程绑定自己的值,可以将ThreadLocal类比喻成全局存放数据的盒子,盒子中可以存储每个线程的私有数据。ThreadLocal解决的是变量在不同线程键的隔离性,也就是不同 线程拥有自己的值,不同线程中的值是可以放入ThreadLocal类中进行保存的。
package org.github.lujiango;public class Test03 {public static ThreadLocal<Object> tl = new ThreadLocal<Object>();static class ThreadA extends Thread {@Overridepublic void run() {try {tl.set("ThreadA");Thread.sleep(200);System.out.println("ThreadA get value = " + tl.get());} catch (Exception e) {e.printStackTrace();}}}static class ThreadB extends Thread {@Overridepublic void run() {try {tl.set("ThreadB");Thread.sleep(200);System.out.println("ThreadB get value = " + tl.get());} catch (Exception e) {e.printStackTrace();}}}public static void main(String[] args) {try {ThreadA a = new ThreadA();ThreadB b = new ThreadB();a.start();b.start();} catch (Exception e) {e.printStackTrace();}}}
第一次调用ThreadLocal类的get()方法返回值是null,可以自定义protected T initialValue()方法,设置默认值。
package org.github.lujiango;class ThreadLocalExt extends ThreadLocal<String> {@Overrideprotected String initialValue() {return "defalut";}
}public class Test03 {public static ThreadLocalExt tl = new ThreadLocalExt();static class ThreadA extends Thread {@Overridepublic void run() {try {System.out.println("ThreadA get value = " + tl.get());tl.set("ThreadA");Thread.sleep(200);System.out.println("ThreadA get value = " + tl.get());} catch (Exception e) {e.printStackTrace();}}}static class ThreadB extends Thread {@Overridepublic void run() {try {System.out.println("ThreadB get value = " + tl.get());tl.set("ThreadB");Thread.sleep(200);System.out.println("ThreadB get value = " + tl.get());} catch (Exception e) {e.printStackTrace();}}}public static void main(String[] args) {try {ThreadA a = new ThreadA();ThreadB b = new ThreadB();a.start();b.start();} catch (Exception e) {e.printStackTrace();}}}
InheritableThreadLocal
InheritableThreadLocal可以在子线程中取得父线程继承下来的值,即可以让子线程从父进程中取得值。
package org.github.lujiango;public class Test04 {static InheritableThreadLocal<String> itl = new InheritableThreadLocal<String>();static class ThreadA extends Thread {@Overridepublic void run() {try {System.out.println("ThreadA get: " + itl.get());} catch (Exception e) {e.printStackTrace();}}}public static void main(String[] args) {try {itl.set("Main");Thread.sleep(1000);ThreadA a = new ThreadA();a.start();} catch (Exception e) {e.printStackTrace();}}}
InheritableThreadLocal即可以设置默认值,也可以在继承父线程值的同时修改。
package org.github.lujiango;class InheritableThreadLocalExt extends InheritableThreadLocal<String> {@Overrideprotected String initialValue() {return "defalut";}@Overrideprotected String childValue(String parentValue) {return parentValue + " child";}
}public class Test04 {static InheritableThreadLocalExt itl = new InheritableThreadLocalExt();static class ThreadA extends Thread {@Overridepublic void run() {try {Thread.sleep(1000);System.out.println("ThreadA get: " + itl.get());} catch (Exception e) {e.printStackTrace();}}}public static void main(String[] args) {try {System.out.println(itl.get());itl.set("Main1");Thread.sleep(1000);ThreadA a = new ThreadA();a.start();itl.set("Main2");System.out.println(itl.get());} catch (Exception e) {e.printStackTrace();}}}
注:如果子线程在取得值得同时,主线程将InheritableThreadLocal中的值进行更改,那么子线程取到的值还是旧值。