Java ThreadLocal原理解析以及应用场景分析案例详解

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public class ThreadLocalTest {
    public static void main(String[] args) {
        ThreadLocal<String> threadLocal = new ThreadLocal<>();
        threadLocal.set("张三");

        new Thread(()->{
            threadLocal.set("李四");
            System.out.println("*******"+Thread.currentThread().getName()+"获取到的数据"+threadLocal.get());
        },"线程1").start();
        new Thread(()->{
            threadLocal.set("王二");
            System.out.println("*******"+Thread.currentThread().getName()+"获取到的数据"+threadLocal.get());
        },"线程2").start();
        new Thread(()->{
            System.out.println("*******"+Thread.currentThread().getName()+"获取到的数据"+threadLocal.get());
        },"线程3").start();
        System.out.println("线程=" + Thread.currentThread().getName() + "获取到的数据=" + threadLocal.get());
    }
}

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public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

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public class Thread implements Runnable {
	
	//每个线程自己的ThreadLocalMap对象通过ThreadLocal保存下来
  ThreadLocal.ThreadLocalMap threadLocals = null;

  ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
}

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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();
    }

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public class ThreadLocal<T> {
		
	private static AtomicInteger nextHashCode =new AtomicInteger();
	
	//初始的Hash值是0x61c88647
	 private static final int HASH_INCREMENT = 0x61c88647;
		
	//每次调用就原子性的将hash值增加HASH_INCREMENT
	private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }
	
	static class ThreadLocalMap {
		 //Entry继承WeakReference
		  static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

				private static final int INITIAL_CAPACITY = 16;


				 private void setThreshold(int len) {
					threshold = len * 2 / 3;
				}


			void createMap(Thread t, T firstValue) {
				t.threadLocals = new ThreadLocalMap(this, firstValue);
			}

			 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);
				}
			}
	}

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private void set(ThreadLocal<?> key, Object value) {
            Entry[] tab = table;
            int len = tab.length;
			//1.根据ThreadLocal对象的hash值，定位到table中的位置i
            int i = key.threadLocalHashCode & (len-1);
			
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();
				//判断Entry.key等于当前的ThreadLoacl对象key，则覆盖旧值，退出。
                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();
        }

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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;
        }

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static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

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ThreadLocal<String> threadLocal = new ThreadLocal<>();
        try {
            threadLocal.set("张三");
        } catch (Exception e) {
            threadLocal.remove();
        }

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public class DateUtil {
    private final static Map<String, ThreadLocal<SimpleDateFormat>> sdfMap = new HashMap<>();
	    public final static String Y2M2D2HMS_ = "yyyy/MM/dd HH:mm:ss";
	    private static SimpleDateFormat getsdf(final String pattern) {
        ThreadLocal<SimpleDateFormat> sdfThread = sdfMap.get(pattern);
        if (sdfThread == null) {
            //双重检验,防止sdfMap被多次put进去值,和双重锁单例原因是一样的
            synchronized (DateUtil.class) {
                // 只有Map中还没有这个pattern的sdf才会生成新的sdf并放入map
                // 这里是关键,使用ThreadLocal<SimpleDateFormat>替代原来直接new SimpleDateFormat
                sdfThread = sdfMap.get(pattern);
                if (sdfThread == null) {
                    sdfThread = ThreadLocal.withInitial(() -> new SimpleDateFormat(pattern));
                    sdfMap.put(pattern, sdfThread);
                }

            }
        }
        return sdfThread.get();
    }
	    /**
     * @param date    需要格式化的date
     * @param pattern 给定转换格式
     * @return java.lang.String 时间串
     * @description 按照指定pattern的方式格式化时间
     */
    public static String formatDate(Date date, String pattern) {
        return DateUtil.getsdf(pattern).format(date);
    }
}

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private static ThreadLocal<DecimalFormat> DECIMAL_FORMAT_THREAD_LOCAL = ThreadLocal.withInitial(() -> new DecimalFormat(DECIMAL_FORMAT));

    /**
     * 获取金额格式化的类
     * @return
     */
    public static DecimalFormat getDecimalFormat() {
        return DECIMAL_FORMAT_THREAD_LOCAL.get();
    }