Java中的ThreadLocal线程变量详解

public T get() {
        // 获取到当前线程
        Thread t = Thread.currentThread();
        // 从当前线程t中获取thredLocals属性
        ThreadLocalMap map = getMap(t);
        //如果不为null，就说明已经初始化过，存在值
        if (map != null) {
            // 获取当前ThreadLocal对象为key值(如果不存在,就创建一个ThreadLocal变量)
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                //如果值不为null,就直接返回
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        // 如果当前线程的threadlocals没有被创建，就调用setInitialValue()创建
        return setInitialValue();
    }
ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
private T setInitialValue() {
        // value赋值为初始化方法值，默认为null
        T value = initialValue();
        // 获取到当前线程
        Thread t = Thread.currentThread();
        // 获取当前线程t的threadlocals属性，也就是map集合
        ThreadLocalMap map = getMap(t);
        // 如果map不为null，进行赋值
        if (map != null)
            map.set(this, value);
        else
            //map为null，创建当前线程t的map并赋值
            createMap(t, value);
        return value;
    }
 

public void set(T value) {
        // 获取到当前线程
        Thread t = Thread.currentThread();
        // 获取到当前线程的threadLocals值，即map值
        ThreadLocalMap map = getMap(t);
        // 如果map不等于null，添加值
        if (map != null)
            map.set(this, value);
        // 如果等于null，创建当前线程t的map，并添加值
        else
            createMap(t, value);
    }

public void remove() {
        // 获取当前线程的threadLocals值，即map集合
         ThreadLocalMap m = getMap(Thread.currentThread());
         // map不为null就将ThreadLocal变量(this)移除
         if (m != null)
             m.remove(this);
     }
}

public class ThreadLocalDemo1 {
    private static ThreadLocal<String> threadLocal1 = new ThreadLocal<>();
    private static ThreadLocal<Integer> threadLocal2 = new ThreadLocal<>();
    /**
     * 运行 count个线程,每个线程持有自己独有的 String类型编号
     */
    public void startThreadArray(int count) {
        Thread[] runs = new Thread[count];
        for (int i = 0; i < runs.length; i++) {
            // 赋值编号id
            new ThreadDemo1(i).start();
        }
    }
    /**
     * 线程类：
     */
    public static class ThreadDemo1 extends Thread {
        /**
         * 编号id
         */
        private int codeId;
        public ThreadDemo1(int codeId) {
            this.codeId = codeId;
        }
        @Override
        public void run() {
            String threadName = Thread.currentThread().getName();
            threadLocal1.set("threadLocal1赋值，线程_" + codeId);
            if (codeId == 2) {
                //如果是线程2，设置 threadLocal2变量，值乘以5
                threadLocal2.set(codeId * 5);
            }
            System.out.println(threadName + " -》 threadLocal1 " + threadLocal1.get());
            System.out.println(threadName + " -》 threadLocal2 " + threadLocal2.get());
            // 使用完移除，help GC
            threadLocal1.remove();
            threadLocal2.remove();
        }
    }
    public static void main(String[] args) {
        ThreadLocalDemo1 useDemo = new ThreadLocalDemo1();
        // 启动3个线程
        useDemo.startThreadArray(3);
    }
}

public class ThreadLocalDemo2 {
    /**
     * 初始化 num值。使用时，先通过get方法获取。
     */
    public static ThreadLocal<ThreadLocalDemo2.Number> threadLocalValue = new ThreadLocal<ThreadLocalDemo2.Number>() {
        @Override
        protected Number initialValue() {
            return new Number(0);
        }
    };
    /**
     * 数据类
     */
    private static class Number {
        public Number(int num) {
            this.num = num;
        }
        private int num;
        public int getNum() {
            return num;
        }
        public void setNum(int num) {
            this.num = num;
        }
        @Override
        public String toString() {
            return "Number [num=" + num + "]";
        }
    }
    /**
     * 线程类：
     */
    public static class ThreadDemo2 extends Thread {
        @Override
        public void run() {
            // 如果没有初始化，注意NPE。
            // static修饰的 number时，注释掉这句
            Number number = threadLocalValue.get();
            //每个线程计数加随机数
            Random r = new Random();
            number.setNum(number.getNum() + r.nextInt(100));
            //将其存储到ThreadLocal中
            threadLocalValue.set(number);
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            //打印保存的随机值
            System.out.println(Thread.currentThread().getName() + " -》 " + threadLocalValue.get().getNum());
            threadLocalValue.remove();
            System.out.println(Thread.currentThread().getName() + " remove方法之后 -》 " + threadLocalValue.get().getNum());
        }
    }
    public static void main(String[] args) {
        // 启动5个线程
        for (int i = 0; i < 5; i++) {
            new ThreadDemo2().start();
        }
    }
}