InheritableThreadLocal 继承自ThreadLocal,重写了childValue、getMap、createMap 方法,主要作用是子线程能够读取父线程数据 看下这个类
publicclassInheritableThreadLocal<T>extendsThreadLocal<T> { protectedTchildValue(TparentValue) { returnparentValue; } //返回的是Thread类的inheritableThreadLocals,而ThreadLocal使用的是threadLocals变量ThreadLocalMapgetMap(Threadt) { returnt.inheritableThreadLocals; } voidcreateMap(Threadt, TfirstValue) { t.inheritableThreadLocals=newThreadLocalMap(this, firstValue); } }
示例:
publicclassInheritableThreadLocalTest { privatestaticfinalInheritableThreadLocal<String>threadLocal=newInheritableThreadLocal<String> (); publicstaticvoidmain(String [] args) throwsInterruptedException { threadLocal.set("hello world"); Threadthread=newThread(newRunnable() { publicvoidrun() { //threadLocal.set("son thread");System.out.println("thread:"+threadLocal.get()); } }); thread.start(); thread.join(); System.out.println("main:"+threadLocal.get()); } }
输出:
thread:hello world main:hello world
这里如果我在子线程中set了一个新值,那结果会怎么样?发现父线程的值没有改变
thread:son thread main:hello world
源码剖析
1. 首先从新建子线程开始分析,这里主要就是将父线程的值copy到子线程中
//构造函数 public Thread(Runnable target) { init(null, target, "Thread-" + nextThreadNum(), 0); } //直接跳到,最终的init方法 private void init(ThreadGroup g, Runnable target, String name, long stackSize, AccessControlContext acc, boolean inheritThreadLocals) { if (name == null) { throw new NullPointerException("name cannot be null"); } this.name = name; Thread parent = currentThread(); SecurityManager security = System.getSecurityManager(); //....省略中间部分,看主要的 //获取父线程的inheritableThreadLocals变量,如果不为空就copy父线程中的变量到子线程 if (inheritThreadLocals && parent.inheritableThreadLocals != null) this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals); /* Stash the specified stack size in case the VM cares */ this.stackSize = stackSize; /* Set thread ID */ tid = nextThreadID(); } //ThreadLocal.createInheritedMap方法 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) { return new ThreadLocalMap(parentMap); } //ThreadLocalMap(parentMap) 方法 private ThreadLocalMap(ThreadLocalMap parentMap) { Entry[] parentTable = parentMap.table; int len = parentTable.length; setThreshold(len); //新建一个Entry数组,Entry继承了WeakReference,key为ThreadLocal类型 //这是为了在大数据量的时候,方便GC来回收已经失效的数据 table = new Entry[len]; for (int j = 0; j < len; j++) { Entry e = parentTable[j]; if (e != null) { @SuppressWarnings("unchecked") ThreadLocal<Object> key = (ThreadLocal<Object>) e.get(); if (key != null) { //InheritableThreadLocal 重写了childValue,返回value值 Object value = key.childValue(e.value); Entry c = new Entry(key, value); //计算数组索引位置,使用"线性探测法" int h = key.threadLocalHashCode & (len - 1); //如果当前位置有值,指针需要移到下一个位置,直到找到不为null的位置 while (table[h] != null) h = nextIndex(h, len); table[h] = c; size++; } } } }
2. 子线程获取父线程值分析,看ThreadLocal的get方法
public T get() { Thread t = Thread.currentThread(); //实际调用InheritableThreadLocal类getMap方法,getMap返回的是当前线程的inheritableThreadLocals变量 //每个线程都有,是Thread类的局部变量 ThreadLocalMap map = getMap(t); //如果是null会初始化一个value为null的ThreadLocalMap if (map != null) { //this就是InheritableThreadLocal类,看下getEntry方法 ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) { @SuppressWarnings("unchecked") T result = (T)e.value; return result; } } return setInitialValue(); }
//这里就是从table数组中去获取索引对应的值,这个table已经在new Thread的时候copy了父线程的数据 private Entry getEntry(ThreadLocal<?> key) { int i = key.threadLocalHashCode & (table.length - 1); Entry e = table[i]; if (e != null && e.get() == key) return e; else //如果条件不成立,会循环整个table,并处理key失效的数据 //如果遍历完还没找到,就返回null return getEntryAfterMiss(key, i, e); }
可能的应用场景
分布式系统中,多个系统串联起来⼯作,为了将这些系统中的请求的⽇志能串联起来,可以在最开始制造⼀个 SN,然后再将SN 放到InheritableThreadLocal⾥,后⾯的系统通过⼀些listener之类的,也将 sn 放到 InheritableThreadLocal中,然后再记录⽇志的配置中,将SN 配置默认输出来,就ok了。
总结
1. 子线程能够读取父线程数据,实际原因是新建子线程的时候,会从父线程copy数据
2. InheritableThreadLocal 继承了ThreadLocal,并重写childValue、getMap、createMap,对该类的操作实际上是对线程ThreadLocalMap的操作
其他文章阅读:
ThreadLocal源码分析 https://www.cnblogs.com/dennyzhangdd/p/7978455.html线性探测法