Create 说说 ThreadLocal 的原理.md

blogs/Android/Framework/Interview/技巧心得相关/说说 ThreadLocal 的原理.md

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+---
+说说 ThreadLocal 的原理
+---
+
+1. ThreadLocal 的适用于什么场景？
+2. ThreadLocal 的使用方式是怎样的？
+3. ThreadLocal 的实现原理怎样的？
+
+```java
+private static void prepare(boolean quitAllowed){
+    sThreadLocal.set(new Looper(quitAllowed));
+}
+static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
+public static Looper myLooper(){
+    return sThreadLocal.get();
+}
+```
+
+```java
+private static final ThreadLocal<Choreographer> sThreadInstance = new ThreadLocal<Choreographer>(){
+    @Override
+    protected Choreographer initialValue(){
+        Looper looper = Looper.myLooper();
+        return new Choreographer(looper);
+    }
+}
+public static Choreographer myLooper(){
+    return sThreadLocal.get();
+}
+```
+
+```java
+public T get(){
+    Thread currentThread = Thread.currentThread();
+    Values values = values(currentThread);
+    if(values!=null){
+        Object[] table = values.table;
+        int index = hash & values.mask;
+        if(this.reference == table[index]){
+            return (T)table[index + 1];
+        }else{
+            values = initializeValues(currentThread);
+        }
+    }
+    return (T)values.getAfterMiss(this);
+}
+Object getAfterMiss(ThreadLocal<?> key){
+    Object[] table = this.table;
+    int index = key.hash & mask;
+    Object value = key.initialVaule();
+    table[index] = key.reference;
+    table[index + 1] = value;
+    size++;
+    cleanUp();
+    return value;
+}
+
+public void set(T value){
+    Thread currentThread = Thread.currentThread();
+    Values values = values(currentThread);
+    if(values == null){
+        values = initalizeValues(currentThread);
+    }
+    values.put(this, value);
+}
+```
+
+#### 总结
+
+1. 同一个 ThreadLocal 对象，在不同线程 get 返回不同的 value
+2. Thread 对象里有张表，保存 ThreadLocal 到 value 的映射关系
+3. 这张表是怎么实现的？
+4. 是如何解决 hash 冲突的？