ThreadLocal是什么？
顾名思义，ThreadLocal可以为每个线程独立存储不同的值（线程本地变量）。其意义在于高并发场景下变量被多个线程访问互不影响，有效避免线程安全问题和同步带来的性能开销。当然它也存在一定缺陷，由于每个线程都会创建ThreadLocal变量，就会带来一定的内存消耗；它的思想就是“以空间换时间”。
特殊情况：InhertiableThreadLocal并不是只存储当前线程的值，它默认会集成父类中的值。
ThreadLocal类方法定义
public class ThreadLocal {
public T get();
private T setInitialValue();
public void set(T value);
public void remove();
}
get：用于获取当前线程私有的ThreadLocal变量；
setInitialValue：可以进行重写设置当前ThreadLocal对应的数据，用于第一次调用get时懒加载获取；
set：设置当前Thread的ThreadLocal值；
remove：删除ThreadLocal中的数据；
get方法具体实现
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
* @return the current thread's value of this thread-local
*/
public T get() {
//获取当前线程
Thread t = Thread.currentThread();
//获取Thread类中定义的ThreadLocal.ThreadLocalMap变量
ThreadLocalMap map = getMap(t);
//判断map是否为null
if (map != null) {
//获取当前ThreadLocal对应的Entry
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
//获取set的值
@SuppressWarnings("unchecked")
T result = (T) e.value;
return result;
}
}
//当map为null或未设置值时调用
return setInitialValue();
}
其中ThreadLocalMap是ThreadLocal中的静态内部类，内部维护了一个Entry数组的table，可以看作是一个kv的map，只不过key是当前ThreadLocal生成的Hash值；
static class ThreadLocalMap {
private Entry[] table;
static class Entry extends WeakReference> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal k, Object v) {
super(k);
value = v;
}
}
}
setInitialValue具体实现
当get方法中map为null或未设置值时会调用setInitialValue方法，接下来看看它的实现：
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
* @return the initial value
*/
private T setInitialValue() {
//value等于重写initialValue后返回的值，否则默认返回null
T value = initialValue();
//获取当前线程
Thread t = Thread.currentThread();
//获取当前Thread类中定义的ThreadLocal.ThreadLocalMap变量
ThreadLocalMap map = getMap(t);
//map为null直接添加到table中，否则新建map
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
值得注意的是其中initialValue默认是返回null的，当然你也可以选择重写来懒加载数据，当调用get方法的时候才获取，就像这样：
ThreadLocal threadLocal = new ThreadLocal() {
int i = 1;
@Override
protected Integer initialValue() {
return i;
}
};
其中当map为null是会调用createMap方法，将创建的ThreadLocalMap赋值到当前Thread的threadLocals变量，完成关联！
/**
* 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);
}
public
class Thread implements Runnable {
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
}
set方法具体实现
和上面setInitialValue中实现几乎一样，这里就不赘述了...
/**
* 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Thread
Thread t = Thread.currentThread();
//获取当前Thread类中定义的ThreadLocal.ThreadLocalMap变量
ThreadLocalMap map = getMap(t);
//map为null直接添加到table中，否则新建map
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
remove方法具体实现
获取当前线程中的ThreadLocal.ThreadLocalMap，清除table数组中以当前ThreadLocal Hash为key的数据。
/**
* Removes the current thread's value for this thread-local
* variable. If this thread-local variable is subsequently
* {@linkplain #get read} by the current thread, its value will be
* reinitialized by invoking its {@link #initialValue} method,
* unless its value is {@linkplain #set set} by the current thread
* in the interim. This may result in multiple invocations of the
* {@code initialValue} method in the current thread.
* @since 1.5
*/
public void remove() {
//获取当前Thread类中定义的ThreadLocal.ThreadLocalMap变量
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
//删除以当前ThreadLocal为key设置的数据
m.remove(this);
}
关于内存泄露的问题
抛出问题
- 首先ThreadLocal实例被线程的ThreadLocalMap实例持有，也可以看成被线程持有。
- 如果应用使用了线程池，那么之前的线程实例处理完之后出于复用的目的依然存活
所以，ThreadLocal设定的值被持有，导致内存泄露。
结论
首先Entry是继承WeakReference
最后
第一次写源码分析，有不好的地方欢迎指正。
参考资料
https://blog.appoptics.com/introduction-to-java-threadlocal-storage/
https://github.com/decaywood/decaywood.github.io/blob/master/_posts/2016/3/2016-03-15-ThreadLocal-intro.markdown
https://droidyue.com/blog/2016/03/13/learning-threadlocal-in-java/
https://fangjian0423.github.io/2014/11/22/java_threadlocal/