一、概念
ThreadLocal在每个线程中对该变量会创建一个副本，即每个线程内部都会有一个该变量，且在线程内部任何地方都可以使用，线程之间互不影响，这样一来就不存在线程安全问题，也不会严重影响程序执行性能。
二、ThreadLocalMap
线程实例的局部变量，用于存储该线程下的ThreadLocal变量
Thread.java
public class Thread implements Runnable {
ThreadLocal.ThreadLocalMap threadLocals = null;
}
ThreadLocalMap.class
static class ThreadLocalMap {
private static final int INITIAL_CAPACITY = 16;
private ThreadLocal.ThreadLocalMap.Entry[] table;
private int size;
private int threshold;
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
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);
}
}
·1、ThreadLocalMap下保存table数组，该数组用于存储当前线程下的所有ThreadLocal变量。且Entry为弱引用，因此不怕内存泄漏问题。
·2、threshold为长度阈值，超过该值则需要扩容table，阈值计算为当前长度的2/3。
1、set（）
private void set(ThreadLocal<?> key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
//TODO 步骤1
if (k == key) {
e.value = value;
return;
}
//TODO 步骤2
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
//TODO 步骤3
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
·1、set（）方法中，会遍历当前的table，首先判断传入的key值是否存在，如果是则更新entry.value然后return
·2、如果找到当前的entry不为空但key为空（可能是ThreadLocal被回收了）则表示该数据无用，则通过replaceStaleEntry更新数据然后return
·3、遍历后无发现合适的位置则插入新Entry，插入后对当前table做清除扩容操作
（1）cleanSomeSlots（）主要就是将那些key不为null,但是value为空的调用expungeStaleEntry(i)清除掉，如果清除了就是返回ture否则返回false
（2）rehash()就是先调用expungeStaleEntry()，然后再根据if (size >= threshold - threshold / 4)去决定要不要resize()
（3）reszie()扩容，清除无用数据，同时生成新key值与生成新阈值【不想看了。】
2、getEntry（）
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);
}
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
ThreadLocal<?> k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
·1、get的逻辑很好懂，首先通过ThreadLocal的换算哈希值判断当前位置是否就是所要的结果，如果不是则以该哈希值为原点遍历table，遍历过程中同时清除不合理的Entry，如果无值则返回null。
三、ThreadLocal
ThreadLocal通过从Thread中获取threadLocals，为ThreadLocalMap对象，该map以当前ThreadLocal为key来保存存储的值。
image.png
1、set()
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
2、get()
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();
}
/**
* 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() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
protected T initialValue() {
return null;
}
·1、首先查询ThreadLocalMap中是否有该值，如果entry不为null则返回entry.value
·2、如果当前map为null或entry为null，则为该map设置初始值。在setInitialValue中会创建ThreadLocalMap，并通过initialValue()获取value的默认值，该方法个人认为是留下一个可扩展的地方。
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