ThreadLocal 简单分析

最新推荐文章于 2025-11-26 15:59:01 发布

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本文深入解析了ThreadLocal的工作原理及其实现细节，包括其内部的ThreadLocalMap结构及其存储和检索机制。同时提供了一个ThreadLocalCache的具体实现案例。

ThreadLocal<T> 是一个泛型类

protected T initialValue() { return null; }// 用于初始化

   private final ThreadLocal<Map<Object, Object>> store;

    public ThreadLocalCache(URL url) {
        this.store = new ThreadLocal<Map<Object, Object>>() {
            @Override
            protected Map<Object, Object> initialValue() {
                return new HashMap<Object, Object>();
            }
        };
    }

ThreadLocal 里面有一个ThreadLocalMap class，这个Map通过 Thread的 threadLocalHashCode和nextHashCode 值算出 hashCode

Thread存储一个字段：保存了ThreadLocalMap

t.threadLocals = new ThreadLocalMap(this, firstValue); // this = ThreadLocal实例

ThreadLocalMap 取值判断

   private Entry getEntry(ThreadLocal key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key) // 不同的ThreadLocal 即使hashCode一样 也不会取到相同的值   
               return e;
            else 
                return getEntryAfterMiss(key,i,e);

ThreadLocal的remove get set 其实就是对Thread实例的字段操作

附：别人的代码

public class ThreadLocalCache implements Cache {

    private final ThreadLocal<Map<Object, Object>> store;

    public ThreadLocalCache(URL url) {
        this.store = new ThreadLocal<Map<Object, Object>>() {
            @Override
            protected Map<Object, Object> initialValue() {
                return new HashMap<Object, Object>();
            }
        };
    }

    public void put(Object key, Object value) {
        store.get().put(key, value);
    }

    public Object get(Object key) {
        return store.get().get(key);
    }

}