ThreadLocal

from: ThreadLocal explained

• java.lang.Thread

    public class Thread implements Runnable {
    // 每個thread 都有一個ThreadLocalMap 來保存自己的ThreadLocal變數
    ThreadLocal.ThreadLocalMap threadLocals = null;
    }
  

• java.lang.ThreadLocal

    // 1. 取得當前Thread實體
    // 2. 取得ThreadLocalMap 
    // 3. 如果ThreadLocalMap或threadlocal是null(第一次呼叫)，設定初始值
    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();
    }
  
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
  
    // 1. 取得初始值
    // 2. 取得ThreadLocalMap
    // 3. 存入value
    //   a. 如果ThreadLocalMap是null, 建立map
    //   b. 直接將(threadlocal, value)存入map
    // 4. 回值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;
    }
  
    // 預設為null，可被overwirte
    protected T initialValue() {
        return null;
    }
  
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }
  
    // 1. 取得當前Thread
    // 2. 取得ThreadLocalMap
    // 3. 存入value 
    //   a. 如果ThreadLocalMap是null, 建立map
    //   b. 將(threadlocal, value)存入map
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
  

• ThreadLocal.ThreadLocalMap

    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);
    }
  
    static class Entry extends WeakReference<ThreadLocal<?>> {
        Object value;
  
        Entry(ThreadLocal<?> k, Object v) {
            super(k);
            value = v;
        }
    }
  
    private Entry[] table;
    private Entry getEntry(ThreadLocal<?> key){...};
    private void set(ThreadLocal<?> key, Object value){...};
    private void remove(ThreadLocal<?> key){...};
  

from:ThreadLocal原理解析（1）：數據存取

Ref

• 《Java源碼分析》：ThreadLocal /ThreadLocalMap
• ThreadLocal explained
• ThreadLocal原理解析（1）：數據存取