public class MyThreadLocal {
    private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
    public static String date(int seconds) {
        Date date = new Date(1000 * seconds);
        return sdf.format(date);
    }

    public static void main(String[] args) {
        // 堆代码 duidaima.com
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 1000; i++) {
            int finalI = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    String date = date(finalI);
                    System.out.println(date);
                }
            });
        }
        executorService.shutdown();
    }
}

public class MyThreadLocal {

    private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");

    public static String date(int seconds) {
        Date date = new Date(1000 * seconds);
        String s = null;
        synchronized (MyThreadLocal.class) {
            s = sdf.format(date);
        }
//        Date date = new Date(1000 * seconds);
        return s;
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 1000; i++) {
            int finalI = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    String date = date(finalI);
                    System.out.println(date);
                }
            });
        }
        executorService.shutdown();
    }
}

public class MyThreadLocal1 {

//    private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
        private static ThreadLocal<SimpleDateFormat> simpleDateFormatThreadLocal = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd hh:mm:ss"));

    public static String date(int seconds) {
        Date date = new Date(1000 * seconds);
        SimpleDateFormat sdf = simpleDateFormatThreadLocal.get();
        return sdf.format(date);
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 1000; i++) {
            int finalI = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    String date = date(finalI);
                    System.out.println(date);
                }
            });
        }
        executorService.shutdown();
    }
}

/**
 * 避免传递参数的麻烦
 * ThreadLocalan案例2
 * @ 堆代码 duidaima.com
 * @since 1.0.0
 */
public class ThreadLocalNormalUsage06 {
    public static void main(String[] args) {
        new Service1().process();
    }
}

class Service1 {
    public void process() {
        User user = new User("周星驰");
        UserContextHolder.holder.set(user);
        new Service2().process();
    }
}

class Service2 {
    public void process() {
        User user = UserContextHolder.holder.get();
        System.out.println("service2:" + user.name);
        UserContextHolder.holder.remove();
        UserContextHolder.holder.set(new User("古天乐"));
        new Service3().process();
    }
}

class Service3 {
    public void process() {
        User user = UserContextHolder.holder.get();
        System.out.println("service3:" + user.name);
    }
}

class UserContextHolder {
    public static ThreadLocal<User> holder
            = new ThreadLocal<>();
}

class User {
    String name;
    public User(String name) {
        this.name = name;
    }
}

.在任何方法中都可以直接获取到对象；

/**
     * Returns the current thread's "initial value" for this
     * thread-local variable.  This method will be invoked the first
     * time a thread accesses the variable with the {@link #get}
     * method, unless the thread previously invoked the {@link #set}
     * method, in which case the {@code initialValue} method will not
     * be invoked for the thread.  Normally, this method is invoked at
     * most once per thread, but it may be invoked again in case of
     * subsequent invocations of {@link #remove} followed by {@link #get}.
     *
     * <p>This implementation simply returns {@code null}; if the
     * programmer desires thread-local variables to have an initial
     * value other than {@code null}, {@code ThreadLocal} must be
     * subclassed, and this method overridden.  Typically, an
     * anonymous inner class will be used.
     *
     * @return the initial value for this thread-local
     */
    protected T initialValue() {
        return null;
    }

    /**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    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();
    }

/* ThreadLocal values pertaining to this thread. This map is maintained by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
    /**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

static class ThreadLocalMap {
        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

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

/**
     * Sets the current thread's copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the {@link #initialValue}
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread's copy of
     *        this thread-local.
     */
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

/**
     * Removes the current thread's value for this thread-local
     * variable.  If this thread-local variable is subsequently
     * {@linkplain #get read} by the current thread, its value will be
     * reinitialized by invoking its {@link #initialValue} method,
     * unless its value is {@linkplain #set set} by the current thread
     * in the interim.  This may result in multiple invocations of the
     * {@code initialValue} method in the current thread.
     *
     * @since 1.5
     */
     public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

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

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

public class ThreadLocalNPE {

   ThreadLocal<Long> tl =  new ThreadLocal();

   public void set(){
       tl.set(Thread.currentThread().getId());
   }
   public long get(){
       return tl.get();
   }

    public static void main(String[] args) {
        ThreadLocalNPE item = new ThreadLocalNPE();
        System.out.println(item.get());

        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                item.set();
                System.out.println(item.get());
            }
        });
        thread.start();
    }
}