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 {
@Override
public 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 {
@Override
public 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> {
@Override
protected String initialValue() {
return "defalut";
}
}public class Test03 {
public static ThreadLocalExt tl = new ThreadLocalExt(); static class ThreadA extends Thread {
@Override
public 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 {
@Override
public 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 {
@Override
public 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> {
@Override
protected String initialValue() {
return "defalut";
} @Override
protected String childValue(String parentValue) {
return parentValue + " child";
}
}public class Test04 {
static InheritableThreadLocalExt itl = new InheritableThreadLocalExt(); static class ThreadA extends Thread {
@Override
public 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。