ThreadLocal工作原理及用法案例

public class ThreadLocalTest {

    //第一种初始化方式
    /**
     * 声明为static是让ThreadLocal实例随着程序的结束才结束，这样才不会让GC回收了
     * 声明为final是让ThreadLocal实例引用不会被替换，这样子也不会因为被替换导致被GC回收
     * 这两个声明都是为了避免作为key的ThreadLocal对象没有外部强引用而导致被GC回收，从而导致内存泄漏的问题，因为ThreadLocalMap<ThreadLocal, Object>中的ThreadLocal
     * 对象作为key是弱引用，会被GC回收。
     */
    private static final ThreadLocal<String> threadLocalStr = ThreadLocal.withInitial(() -> "fresh");

    private static AtomicInteger intGen = new AtomicInteger(0);
    //第二种初始化方式
    private static final ThreadLocal<Integer> threadLocalInt = new ThreadLocal<Integer>() {
        @Override
        public Integer initialValue() {
            return intGen.incrementAndGet();
        }
    };

    public static void main(String[] args) throws InterruptedException {
        ArrayList<Thread> threads = new ArrayList<>();
        for (int i = 0; i < 2; i++) {
            Thread t = new Thread(() -> {
                try {
                    System.out.println(Thread.currentThread().getName() + "   " + threadLocalInt.get());
                    System.out.println(Thread.currentThread().getName() + "   " + threadLocalStr.get());
                    TimeUnit.SECONDS.sleep(5);
                    threadLocalStr.set("bojack horseman" + threadLocalInt.get());
                    System.out.println(Thread.currentThread().getName() + "   " + threadLocalInt.get());
                    System.out.println(Thread.currentThread().getName() + "   " + threadLocalStr.get());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    threadLocalInt.remove();
                    threadLocalStr.remove();
                }
            });
            t.start();
            threads.add(t);
        }
        TimeUnit.SECONDS.sleep(2);
        System.out.println(threads);
        System.out.println(threadLocalStr);
        System.out.println(threadLocalInt);
    }
    /**
     * Thread-0   1
     * Thread-1   2
     * Thread-0   fresh
     * Thread-1   fresh
     * [Thread[Thread-0,5,main], Thread[Thread-1,5,main]]
     * java.lang.ThreadLocal$SuppliedThreadLocal@1ef7fe8e
     * cn.vv.schedule.test.ThreadLocalTest$1@6f79caec
     * Thread-1   2
     * Thread-1   bojack horseman2
     * Thread-0   1
     * Thread-0   bojack horseman1
     */

}

public class InheritableThreadLocalTest {

    //第一种初始化方式
    private static final InheritableThreadLocal<String> threadLocalStr = new InheritableThreadLocal<String>() {
        @Override
        public String initialValue() {
            return "fresh";
        }
    };
    private static AtomicInteger intGen = new AtomicInteger(0);
    //第二种初始化方式
    private static final ThreadLocal<Integer> threadLocalInt = new ThreadLocal<Integer>() {
        @Override
        public Integer initialValue() {
            return intGen.incrementAndGet();
        }
    };

    public static void main(String[] args) throws InterruptedException {
        //如果是InheritableThreadLocal，则父线程创建的所有子线程都会复制一份父线程的线程变量，而不是去初始化一份线程变量
        threadLocalStr.set("main");
        ArrayList<Thread> threads = new ArrayList<>();
        for (int i = 0; i < 2; i++) {
            Thread t = new Thread(() -> {
                try {
                    System.out.println(Thread.currentThread().getName() + "   " + threadLocalInt.get());
                    System.out.println(Thread.currentThread().getName() + "   " + threadLocalStr.get());
                    TimeUnit.SECONDS.sleep(5);
                    //子线程可以自由地改变自己的本地变量
                    threadLocalStr.set("bojack horseman" + threadLocalInt.get());
                    System.out.println(Thread.currentThread().getName() + "   " + threadLocalInt.get());
                    System.out.println(Thread.currentThread().getName() + "   " + threadLocalStr.get());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    threadLocalInt.remove();
                    threadLocalStr.remove();
                }
            });
            t.start();
            threads.add(t);
        }
        TimeUnit.SECONDS.sleep(2);
        System.out.println(Thread.currentThread().getName() + "   " + threadLocalStr.get());
    }
    /**
     * Thread-0   2
     * Thread-1   1
     * Thread-0   main
     * Thread-1   main
     * main   main
     * Thread-0   2
     * Thread-0   bojack horseman2
     * Thread-1   1
     * Thread-1   bojack horseman1
     */

}