/**
 * 画布类
 */
public class Canvas {
	
	private String content;

	public String getContent() {
		return content;
	}

	public void setContent(String content) {
		this.content = content;
	}
}

/**
 * 画家类
 */
public class Painter extends Thread {

	private String name;
	
	private Canvas canvas;
	
	private String color;
	
	public Painter(String name, Canvas canvas, String color) {
		this.name = name;
		this.canvas = canvas;
		this.color = color;
	}

	@Override
	public void run() {
		canvas.setContent(color);
		System.out.println(this.name + "在画板绘制" + canvas.getContent());
		
	}
}

/**
 * 启动类
 */
public class Demo {

	public static void main(String[] args) {
		
		// 创建画布
		Canvas canvas = new Canvas();
		
		Painter painter1 = new Painter("小强", canvas, "红色");
		Painter painter2 = new Painter("旺财", canvas, "黄色");
		Painter painter3 = new Painter("狗蛋", canvas, "蓝色");
		
		painter1.start();
		painter2.start();
		painter3.start();
		
	}
}

public class Canvas {
	
	private ThreadLocal<String> map = new ThreadLocal();

	public String getContent() {
		return map.get();
	}

	public void setContent(String content) {
		map.set(content);
	}
}

public void set(T value) {
    // 获取当前线程对象
    Thread t = Thread.currentThread();
    // 获取当前线程对象维护的 ThreadLocalMap 对象
    ThreadLocalMap map = getMap(t);
    if (map != null)
        // 如果 map 存在设置 entry
        map.set(this, value);
    else
        // 如果 map 不存在，由于 threadLocal 实例帮忙创建并绑定数据
        createMap(t, value);
}

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

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

public T get() {
    // 获取当前线程对象
    Thread t = Thread.currentThread();
    // 获取当前线程对象维护的 ThreadLocalMap 对象
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        // 如果 map 不为空，以当前的 ThreadLocal 实例为 key, 获取数据
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    // 如果 map 为空，初始化值，通常为 null
    return setInitialValue();
}

private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

protected T initialValue() {
    return null;
}

public void remove() {
     ThreadLocalMap m = getMap(Thread.currentThread());
     if (m != null)
         m.remove(this);
 }

/**
 * 初始容量，必须是 2 的整次幂
 */
private static final int INITIAL_CAPACITY = 16;

/**
 * 存放数据的 table，数据长度也是 2 的整次幂
 */
private Entry[] table;

/**
 * 数组中 entry 的个数
 */
private int size = 0;

/**
 * 进行扩展的阀值
 */
private int threshold; // Default to 0

static class Entry extends WeakReference<ThreadLocal<?>> {

    Object value;

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

当 ThreadLocal 使用结束，栈内存的 ThreadLocal 引用被回收，即引用 1 不再指向 ThreadLocal 对象。

由于引用 2 是弱引用，没有任何强引用指向 ThreadLocal 对象，因此 ThreadLocal 对象会被 GC 回收，此时 Entry 的 key = null

如果我们没有会手动删除 Entry 对象，且当前线程一直在运行中，会存在一个强引用链 Thread 引用-> Thread 对象-> ThreadLocal 对象-> Entry 对象 -> Value，由于 value 不会被回收，而 key 又为 null, value 这块内存就永远无法被访问，这就造成了内存泄漏，