Java子线程获取主线程的全局变量(ThreadLocal)

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);     }