ThreadLocal源码分析

电玩女神 2022-12-08 05:08 158阅读 0赞

### jdk1.8中ThreadLocal源码解读 ###

为了方便阅读调整了部分代码的顺序，整体结构没变。

public class ThreadLocal<T> { 
    
        private final int threadLocalHashCode = nextHashCode(); // 初始hash码常量
        private static AtomicInteger nextHashCode = new AtomicInteger(); // hash码的原子变量
        private static final int HASH_INCREMENT = 0x61c88647;  // hash码增量
    
        private static int nextHashCode() { 
            return nextHashCode.getAndAdd(HASH_INCREMENT); // 返回原子变量当前值，并+=hash增量
        }
    
    	/* * 传入的是一个函数式接口Supplier（生产者，需要掌握java8的lombda表达式）， * 其作用是返回一个自定义的初始值 */
        public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) { 
            return new SuppliedThreadLocal<>(supplier);
        }
    
    	// ThreadLocal 的构造方法
        public ThreadLocal() { }
    
    	// 获取线程绑定变量的值
        public T get() { 
            Thread t = Thread.currentThread(); // 获取当前线程
            ThreadLocalMap map = getMap(t); // 获取当前线程绑定的ThreadLocalMap（和HashMap差不多，就是用来存数据的） 
            if (map != null) {  // 如果当前线程存在map
                ThreadLocalMap.Entry e = map.getEntry(this); // 获取map中的 key
                if (e != null) {   // 如果key存在
                    @SuppressWarnings("unchecked")
                    T result = (T)e.value;  //获取key所绑定的value数据
                    return result; // 将结果返回
                }
            }
            return setInitialValue();//如果map不存在则创建一个map并且设置初始值
        }
        
    	// 默认初始值为null
        protected T initialValue() { return null;}
        //设置初始值
        private T setInitialValue() { 
            T value = initialValue(); // null
            Thread t = Thread.currentThread(); // 获取当前线程
            ThreadLocalMap map = getMap(t); // 获取当前线程所绑定的 ThreadLocalMap
            if (map != null) // map不为null说明当前线程存了数据
                map.set(this, value); // 将初始值null加入当前map中
            else
                createMap(t, value); //map为null，说明当前线程没有map，创建一个map绑定到当前线程
            return value; // 返回 初始值null
        }
    	//设置值
        public void set(T value) { 
            Thread t = Thread.currentThread();// 获取当前线程
            ThreadLocalMap map = getMap(t); // 获取当前线程所绑定的 ThreadLocalMap
            if (map != null)
                map.set(this, value); //将当前值value覆盖map中原先的值
            else
                createMap(t, value); // 创建map，并且将当前值value 添加到map中
        }
    
    	// 删除map中存的数据
        public void remove() { 
             ThreadLocalMap m = getMap(Thread.currentThread()); // 获取当前线程绑定的map
             if (m != null)
                 m.remove(this);// 移除数据
         }
    
        ThreadLocalMap getMap(Thread t) { 
            return t.threadLocals; // 返回Thread类中的ThreadLocalMap（需要看Thread类源码）
        }
    
    
        void createMap(Thread t, T firstValue) { 
            t.threadLocals = new ThreadLocalMap(this, firstValue);//创建一个map并绑定到当前线程
        }
    
    
        static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) { 
            return new ThreadLocalMap(parentMap);// 创建一个内嵌的Map，将parentMap嵌入到当前新的Map
        }
    
    
        T childValue(T parentValue) { 
            throw new UnsupportedOperationException(); // 抛异常，不支持当前操作异常
        }
    
    	// 静态常量内部类
        static final class SuppliedThreadLocal<T> extends ThreadLocal<T> { 
        	/* * 这里的代码省略不写，意思就是返回一个初始值 */
        }
    
    	// 静态内部类，后面单独将源码列出
    	static class ThreadLocalMap{ 
    		/* * 这部分代码和HashMap很相似 */
    	}
    }

--------------------

### ThreadLocalMap源码解读 ###

#### 简化版本 ####

static class ThreadLocalMap { 
    	/*下面两个都是构造方法*/
        ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) { }
        private ThreadLocalMap(ThreadLocalMap parentMap) { }
        
    	/*存数据的数据结构*/
        static class Entry extends WeakReference<ThreadLocal<?>> { 
            Object value;
            Entry(ThreadLocal<?> k, Object v) { 
                super(k);
                value = v;
            }
        }
        /* * 重要！！真正存储数据的位置，后面set方法就是在这个位置存入数据 */
        private Entry[] table; // 用来存数据的对象数组，每一个Entry有一个value属性用来存数据
        
        private static final int INITIAL_CAPACITY = 16; // map的初始容量16和HashMap一样
        private int size = 0; // map的初始大小为0
        private int threshold; // 触发扩容的大小，与hashMap一致的扩容机制
        private void setThreshold(int len) { }  // 设置触发扩容的大小
        
        private static int nextIndex(int i, int len) { } // 返回Entry数组中下一个的hash索引
        private static int prevIndex(int i, int len) { } // 返回Entry数组中上一个的hash索引
        private Entry getEntry(ThreadLocal<?> key) { } // 获得map的key
    
        private void set(ThreadLocal<?> key, Object value) { } // 将数据存入到Entry数组中
        private void remove(ThreadLocal<?> key) { } //将数据从Entry数组中移除
    
        private void rehash() { }  // hash冲撞，进行重新hash
        private void resize() { }  // 扩容，与hashMap一样 
    
    	/* * 下面的方法对于开发来说不是很重要。 */
        private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) { }
        private void replaceStaleEntry(ThreadLocal<?> key, Object value,int staleSlot) { }
        private int expungeStaleEntry(int staleSlot) { }
        private boolean cleanSomeSlots(int i, int n) { }
        private void expungeStaleEntries() { }
    }

#### 完整源码（不包含注释） ####

static class ThreadLocalMap { 
    
        static class Entry extends WeakReference<ThreadLocal<?>> { 
            Object value;
            Entry(ThreadLocal<?> k, Object v) { 
                super(k);
                value = v;
            }
        }
    
      
        private static final int INITIAL_CAPACITY = 16;
        private Entry[] table;
        private int size = 0;
        private int threshold; // Default to 0
    
    
        private void setThreshold(int len) { 
            threshold = len * 2 / 3;
        }
    
        private static int nextIndex(int i, int len) { 
            return ((i + 1 < len) ? i + 1 : 0);
        }
    
        private static int prevIndex(int i, int len) { 
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }
    
        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);
        }
    
    
        private ThreadLocalMap(ThreadLocalMap parentMap) { 
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];
            for (int j = 0; j < len; j++) { 
                Entry e = parentTable[j];
                if (e != null) { 
                    @SuppressWarnings("unchecked")
                    ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
                    if (key != null) { 
                        Object value = key.childValue(e.value);
                        Entry c = new Entry(key, value);
                        int h = key.threadLocalHashCode & (len - 1);
                        while (table[h] != null)
                            h = nextIndex(h, len);
                        table[h] = c;
                        size++;
                    }
                }
            }
        }
    
        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;
        }
    
        private void set(ThreadLocal<?> key, Object value) { 
            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();
    
                if (k == key) { 
                    e.value = value;
                    return;
                }
    
                if (k == null) { 
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
    
            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }
    
    
        private void remove(ThreadLocal<?> key) { 
            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)]) { 
                if (e.get() == key) { 
                    e.clear();
                    expungeStaleEntry(i);
                    return;
                }
            }
        }
    
        private void replaceStaleEntry(ThreadLocal<?> key, Object value,
                                       int staleSlot) { 
            Entry[] tab = table;
            int len = tab.length;
            Entry e;
            int slotToExpunge = staleSlot;
            for (int i = prevIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = prevIndex(i, len))
                if (e.get() == null)
                    slotToExpunge = i;
    
            for (int i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) { 
                ThreadLocal<?> k = e.get();
    
                if (k == key) { 
                    e.value = value;
    
                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;
    
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }
    
            tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);
    
            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }
    
        private int expungeStaleEntry(int staleSlot) { 
            Entry[] tab = table;
            int len = tab.length;
    
            tab[staleSlot].value = null;
            tab[staleSlot] = null;
            size--;
    
            Entry e;
            int i;
            for (i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) { 
                ThreadLocal<?> k = e.get();
                if (k == null) { 
                    e.value = null;
                    tab[i] = null;
                    size--;
                } else { 
                    int h = k.threadLocalHashCode & (len - 1);
                    if (h != i) { 
                        tab[i] = null;
                        while (tab[h] != null)
                            h = nextIndex(h, len);
                        tab[h] = e;
                    }
                }
            }
            return i;
        }
    
        
        private boolean cleanSomeSlots(int i, int n) { 
            boolean removed = false;
            Entry[] tab = table;
            int len = tab.length;
            do { 
                i = nextIndex(i, len);
                Entry e = tab[i];
                if (e != null && e.get() == null) { 
                    n = len;
                    removed = true;
                    i = expungeStaleEntry(i);
                }
            } while ( (n >>>= 1) != 0);
            return removed;
        }
    
    
        private void rehash() { 
            expungeStaleEntries();
    
            // Use lower threshold for doubling to avoid hysteresis
            if (size >= threshold - threshold / 4)
                resize();
        }
    
    
        private void resize() { 
            Entry[] oldTab = table;
            int oldLen = oldTab.length;
            int newLen = oldLen * 2;
            Entry[] newTab = new Entry[newLen];
            int count = 0;
    
            for (int j = 0; j < oldLen; ++j) { 
                Entry e = oldTab[j];
                if (e != null) { 
                    ThreadLocal<?> k = e.get();
                    if (k == null) { 
                        e.value = null; // Help the GC
                    } else { 
                        int h = k.threadLocalHashCode & (newLen - 1);
                        while (newTab[h] != null)
                            h = nextIndex(h, newLen);
                        newTab[h] = e;
                        count++;
                    }
                }
            }
    
            setThreshold(newLen);
            size = count;
            table = newTab;
        }
    
        private void expungeStaleEntries() { 
            Entry[] tab = table;
            int len = tab.length;
            for (int j = 0; j < len; j++) { 
                Entry e = tab[j];
                if (e != null && e.get() == null)
                    expungeStaleEntry(j);
            }
        }
    }