两者什么差异呢,咱们来看下源码中的官方注释?
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals =null;
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
都是Thread的变量,看着是不是几乎一样. 正如inheritableThreadLocals 变量比threadLocals名称多的inheritable.
这里inheritableThreadLocals是可继承的.
这是什么意思呢?:
1)我們知道threadLocals 是线程独有的,也就是线程安全的.我们可将对线程安全 有要求的对象存在这里备用.
2)但是这里使用inheritableThreadLocals,a继承b,则a线程内部的inheritableThreadLocals会持有b中的值.
如何使用呢?
1)我们首先,针对threadLocals 和inheritableThreadLocals, 有两个类:ThreadLocal 和InheritableThreadLocal
2)之前如何使用,将其替换就可以
Thread-A:
ThreadLocal threadLocal = new new ThreadLocal<Object>(); //使用ThreadLocal的方式
Thread-B:
ThreadLocal threadLocal = new new InheritableThreadLocal <Object>();//使用InheritableThreadLocal的方式
这个能力是如何实现的呢?
这个我们要看构造Thread的时候做了什么
//Thread
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
........
if (inheritThreadLocals && parent.inheritableThreadLocals != null) //这里inheritThreadLocals 一直是true
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals); //这里是关键
......
}
ThreadLocal.createInheritedMap 又做了什么呢?
private ThreadLocalMap(ThreadLocalMap parentMap) {
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
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) {
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}
我们看到所有的数值parentMap的值copy了一份到当前的线程table中,我们取得时候回去这里取:
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
return getEntryAfterMiss(key, i, e);
}
其实用的时候,就当做一个普通陌生的黑盒使用就好
