ThreadLocal内存泄漏问题
根据上面Entry方法的源码，我们知道ThreadLocalMap是使用ThreadLocal的弱引用作为Key的。下图是本文介绍到的一些对象之间的引用关系图，实线表示强引用，虚线表示弱引用：
如上图，ThreadLocalMap使用ThreadLocal的弱引用作为key，如果一个ThreadLocal没有外部强引用引用他，那么系统gc的时候，这个ThreadLocal势必会被回收，这样一来，ThreadLocalMap中就会出现key为null的Entry，就没有办法访问这些key为null的Entry的value，如果当前线程再迟迟不结束的话，这些key为null的Entry的value就会一直存在一条强引用链：
Thread Ref -> Thread -> ThreaLocalMap -> Entry -> value
永远无法回收，造成内存泄露。
ThreadLocal的hashcode
在ThreadLocalMap中都需要根据索引i来get,set
int i = key.threadLocalHashCode & (len-1);这里关键的threadLocalHashCode
下面仿照ThreadLocal来跑threadLocalHashCode
单线程，多实例化
public class ThreadLocalMapDemo {
private final int threadLocalHashCode = nextHashCode();
private static AtomicInteger nextHashCode =
new AtomicInteger();
private static final int HASH_INCREMENT = 0x61c88647;
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
public static void main(String[] args) {
System.out.println(new ThreadLocalMapDemo().threadLocalHashCode);
System.out.println(new ThreadLocalMapDemo().threadLocalHashCode);
System.out.println(new ThreadLocalMapDemo().threadLocalHashCode);
System.out.println(new ThreadLocalMapDemo().threadLocalHashCode);
}
}Output:
0
1640531527
-1013904242
626627285多线程，单实例化
public class ThreadLocalMapDemo {
private final int threadLocalHashCode = nextHashCode();
private static AtomicInteger nextHashCode =
new AtomicInteger();
private static final int HASH_INCREMENT = 0x61c88647;
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
public static void main(String[] args) {
for(int i=0;i<5;i++){
new Thread(() -> {
System.out.println("threadName:"+Thread.currentThread().getName()+":"+new ThreadLocalMapDemo().threadLocalHashCode);
}).start();
}
}
}Output:
threadName:Thread-0:0
threadName:Thread-1:1640531527
threadName:Thread-2:-1013904242
threadName:Thread-3:626627285
threadName:Thread-4:-2027808484魔数0x61c88647
参考：
Java进阶（七）正确理解Thread Local的原理与适用场景
ThreadLocal源码详细解析、魔数0x61c88647