2019-08-10-Handler机制之ThreadLocal
ThreadLocal顾名思义就是线程本地变量
1，ThreadLocal介绍
线程局部变量，就是每个线程都有一个自己独一的变量副本，举个例子，大家就能明白
ThreadLocal<String> tl = new ThreadLocal<>();
tl.set("main");
Thread a = new Thread() {
@Override
public void run() {
super.run();
tl.set("A");
System.out.println("a = [" + tl.get() + "]");
}
};
Thread b = new Thread() {
@Override
public void run() {
super.run();
tl.set("B");
System.out.println("b = [" + tl.get() + "]");
}
};
a.start();
b.start();
System.out.println("main = [" + tl.get() + "]");
上面代码的输出结果是
a = [A]
main = [main]
b = [B]
如上可以看出，虽然不同的线程访问了同一个对象tl，但是它们通过tl获取到的对象却是不同的，也就是说ThreadLocal为每个线程保存了一个副本
2，ThreadLocal的原理
1，set的原理
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
从上面的源码可以看出，ThreadLocal.set方法的本质是先通过当前线程获取到当前线程的ThreadLocalMap，然后调用ThreadLocalMap.set(key, value)方法，如果为空的情况下，使用当前线程来创建ThreadLocalMap，如下
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
getMap源码如下：
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
如上可以看出，ThreadLocalMap是线程的属性之一，因此它是和当前线程相关的
2，get原理
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();
}
如上可以看出，尝试从ThreadLocalMap中获取，如果没有获取的，则返回默认null
3，ThreadLocal.ThreadLocalMap原理
static class ThreadLocalMap {
/**
* The entries in this hash map extend WeakReference, using
* its main ref field as the key (which is always a
* ThreadLocal object). Note that null keys (i.e. entry.get()
* == null) mean that the key is no longer referenced, so the
* entry can be expunged from table. Such entries are referred to
* as "stale entries" in the code that follows.
*/
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
/**
* The initial capacity -- MUST be a power of two.
*/
private static final int INITIAL_CAPACITY = 16;
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
/**
* The number of entries in the table.
*/
private int size = 0;
/**
* The next size value at which to resize.
*/
private int threshold; // Default to 0
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
/**
* Construct a new map initially containing (firstKey, firstValue).
* ThreadLocalMaps are constructed lazily, so we only create
* one when we have at least one entry to put in it.
*/
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
}
从上面的源码可以看出，ThreadLocalMap内部仅仅维护了Entry[] table,数组。其中Entry实体中对应的key为弱引用（下文会将为什么会用弱引用），在第一次放入数据时，会初始化数组长度（为16),定义数组扩容阀值（当前默认数组长度的2/3)。
如果key使用强引用，那么当引用ThreadLocal的对象被回收了，但ThreadLocalMap中还持有ThreadLocal的强引用，如果没有手动删除，ThreadLocal不会被回收，导致内存泄漏。
弱引用的key也可能带来一些问题，就是ThreadLocalMap可能有很多null为key的数据，而此时的value依然是被强引用，因此
注意：不管是调用ThreadLocal的set（）还是get()方法，都会去清除key==null的数据。
- ThreadLocal本质是操作线程中ThreadLocalMap来实现本地线程变量的存储的
- ThreadLocalMap是采用数组的方式来存储数据，其中key(弱引用)指向当前ThreadLocal对象，value为设的值
- ThreadLocal为内存泄漏采取了处理措施，在调用ThreadLocal的get(),set(),remove()方法的 时候都会清除线程ThreadLocalMap里所有key为null的Entry
- 在使用ThreadLocal的时候，我们仍然需要注意，避免使用static的ThreadLocal，分配使用了ThreadLocal后，一定要根据当前线程的生命周期来判断是否需要手动的去清理ThreadLocalMap中清key==null的Entry。
