如何解析InheritableThreadLocal
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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的变量 看下这个类
public class InheritableThreadLocalextends ThreadLocal { protected T childValue(T parentValue) { return parentValue; } //返回的是Thread类的inheritableThreadLocals,而ThreadLocal使用的是threadLocals变量 ThreadLocalMap getMap(Thread t) { return t.inheritableThreadLocals; } void createMap(Thread t, T firstValue) { t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue); } }
示例:
public class InheritableThreadLocalTest { private static final InheritableThreadLocalthreadLocal = new InheritableThreadLocal (); public static void main(String [] args) throws InterruptedException { threadLocal.set("hello world"); Thread thread = new Thread(new Runnable() { @Override public void run() { //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
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
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); }
总结
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
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
上述就是小编为大家分享的如何解析InheritableThreadLocal 了,如果刚好有类似的疑惑,不妨参照上述分析进行理解。如果想知道更多相关知识,欢迎关注创新互联行业资讯频道。
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