接上篇spring boot+多线程的使用,引入多线程后很大概率会遇到主线程全局变量共享给子线程的情况,以及各线程变量不共享的情况,下面就简单介绍下两个神器
ThreadLocal
InheritableThreadLocal
ThreadLocal做线程级的数据隔离
InheritableThreadLocal可继承的线程变量表,可以让子线程获取到父线程中ThreadLocal的值。
看个例子:
public class BaseTest {
public static final InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();
public static final ThreadLocal<String> threadLocal = new ThreadLocal<>();public static void main(String[] args) {
inheritableThreadLocal.set("Inheritable hello");
threadLocal.set("hello");
new Thread(()->{
System.out.println(String.format("子线程可继承值:%s",inheritableThreadLocal.get()));
System.out.println(String.format("子线程值:%s",threadLocal.get()));
new Thread(()->{
System.out.println(String.format("孙子线程可继承值:%s",inheritableThreadLocal.get()));
System.out.println(String.format("孙子线程值:%s",threadLocal.get()));
}).start();}).start();
}
执行结果:
可以看到不可继承的ThreadLocal子线程是不能共享父线程的。可继承的ThreadLocal如何实现呢?
其原理和ThreadLocal基本上一致,都是线程中存有ThreadLocalMap
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
我们在new线程时init方法会有如下操作。
Thread parent = currentThread(); ../省略代码 if (inheritThreadLocals && parent.inheritableThreadLocals != null) this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
如果允许new的线程继承当前线程的threadlocalMap,那么new的线程会copy一份当前线程也就是父线程的inheritableThreadLocals 。这儿也可以说明继承有两条件,new的线程允许继承(默认允许),父线程的inheritableThreadLocals 不为null。
**这儿要注意不管是创建ThreadLocal还是inheritableThreadLocals(如果父线程没有) 的ThreadLocalMap都是在Threadlocal.set方法的时候创建的,即懒加载。
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}