ThreadLocal|ThreadLocal 理解 ThreadLocal理解

Thread 变量 threadLocals
线程类Thread内部有一个名为threadLocals 的 ThreadLocal.ThreadLocalMap 变量，它定义如下：

/* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ ThreadLocal.ThreadLocalMap threadLocals = null;

ThreadLocal.ThreadLocalMap
ThreadLocal.ThreadLocalMap 是 ThreadLocal 的内部类

/** * ThreadLocalMap is a customized hash map suitable only for * maintaining thread local values. No operations are exported * outside of the ThreadLocal class. The class is package private to * allow declaration of fields in class Thread.To help deal with * very large and long-lived usages, the hash table entries use * WeakReferences for keys. However, since reference queues are not * used, stale entries are guaranteed to be removed only when * the table starts running out of space. */ static class ThreadLocalMap { // ... }

看注释的第一句话 ThreadLocalMap is a customized hash map suitable only for maintaining thread local values. 意思是 ThreadLocalMap 是一个自定义 HashMap，只适用于线程局部变量。完全可以把它看做是一个 HashMap。

【ThreadLocal|ThreadLocal 理解】也就是说，在 Thread 内部有一个类似 HashMap 的成员变量

ThreadLocal

/** * This class provides thread-local variables.These variables differ from * their normal counterparts in that each thread that accesses one (via its * {@code get} or {@code set} method) has its own, independently initialized * copy of the variable.{@code ThreadLocal} instances are typically private * static fields in classes that wish to associate state with a thread (e.g., * a user ID or Transaction ID). * * For example, the class below generates unique identifiers local to each * thread. * A thread's id is assigned the first time it invokes {@code ThreadId.get()} * and remains unchanged on subsequent calls. * * ... * * @authorJosh Bloch and Doug Lea * @since1.2 */ public class ThreadLocal { //... }

大致意思就是，ThreadLocal 提供线程局部的变量，这些变量在每个线程中都有不同的副本，通过get和set方法就可以得到或设置它们。

类ThreadLocal的结构如下：

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类`ThreadLocal`的结构
重要的方法只有两个：set 和 get
ThreadLocal.set(value)
ThreadLocal.set(value) 向 Thread 内部的 threadLocals 变量添加值，它的实现如下：

// line 189 /** * Sets the current thread's copy of this thread-local variable * to the specified value.Most subclasses will have no need to * override this method, relying solely on the {@link #initialValue} * method to set the values of thread-locals. * * @param value the value to be stored in the current thread's copy of *this thread-local. */ public void set(T value) { // 获取当前线程 Thread t = Thread.currentThread(); // 获取线程内部 ThreadLocalMap 对象 ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else// 给线程threadLocals变量new一个ThreadLocalMap对象，并添加第一个键值对 createMap(t, value); } ...... // line 225 /** * Get the map associated with a ThreadLocal. Overridden in * InheritableThreadLocal. * * @paramt the current thread * @return the map */ ThreadLocalMap getMap(Thread t) { return t.threadLocals; }/** * Create the map associated with a ThreadLocal. Overridden in * InheritableThreadLocal. * * @param t the current thread * @param firstValue value for the initial entry of the map */ void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }

ThreadLocal.get()

public T get() { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) { @SuppressWarnings("unchecked") T result = (T)e.value; return result; } } return setInitialValue(); }

小结

ThreadLocal和Synchronized都是为了解决多线程中相同变量的访问冲突问题，不同的是：

Synchronized 是通过线程等待，牺牲时间来解决访问冲突

ThreadLocal 是通过每个线程单独一份存储空间，牺牲空间来解决冲突，并且相比于Synchronized，ThreadLocal 具有线程隔离的效果，只有在线程内才能获取到对应的值，线程外则不能访问到想要的值。