java ThreadLocal 分析by 源码

private ThreadLocal<StopWatch> stopWatch = new ThreadLocal<>();//定义成员变量
//方法1里
 StopWatch sw = new StopWatch();
 stopWatch.set(sw);//保存当前线程关联变量
//方法2里
 StopWatch sw = stopWatch.get(); //需要的地方取出线程关联变量
 sw.stop();

public void set(T value) {
    Thread t = Thread.currentThread();//获取当前线程
    ThreadLocalMap map = getMap(t);//获取当前线程的threadLocals 变量ThreadLocalMap类型
    if (map != null)
        map.set(this, value);//已创建，就用当前ThreadLocal对象作为key保存到map
    else
        createMap(t, value);//第一次就创建
        }

void createMap(Thread t, T firstValue) {//创建map方法，复制到当前线程的threadLocals 属性里
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

/* ThreadLocal values pertaining to this thread. This map is maintained
 * by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;

/*
 * InheritableThreadLocal values pertaining to this thread. This map is
 * maintained by the InheritableThreadLocal class.
 * 这个是子线程可以继承的ThreadLocal属性,具体使用InheritableThreadLocal 类做存储
 */
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

private InheritableThreadLocal<StopWatch> stopWatch = new InheritableThreadLocal<>();

private void init(ThreadGroup g, Runnable target, String name,
                  long stackSize, AccessControlContext acc,
                  boolean inheritThreadLocals) {
    if (name == null) {
        throw new NullPointerException("name cannot be null");
    }

    this.name = name;

    Thread parent = currentThread();
    SecurityManager security = System.getSecurityManager();
    if (g == null) {
        /* Determine if it's an applet or not */

        /* If there is a security manager, ask the security manager
           what to do. */
        if (security != null) {
            g = security.getThreadGroup();
        }

        /* If the security doesn't have a strong opinion of the matter
           use the parent thread group. */
        if (g == null) {
            g = parent.getThreadGroup();
        }
    }

    /* checkAccess regardless of whether or not threadgroup is
       explicitly passed in. */
    g.checkAccess();

    /*
     * Do we have the required permissions?
     */
    if (security != null) {
        if (isCCLOverridden(getClass())) {
            security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
        }
    }

    g.addUnstarted();

    this.group = g;
    this.daemon = parent.isDaemon();
    this.priority = parent.getPriority();
    if (security == null || isCCLOverridden(parent.getClass()))
        this.contextClassLoader = parent.getContextClassLoader();
    else
        this.contextClassLoader = parent.contextClassLoader;
    this.inheritedAccessControlContext =
            acc != null ? acc : AccessController.getContext();
    this.target = target;
    setPriority(priority);
    if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        this.inheritableThreadLocals =
            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);//这一句复制父类的local变量
    /* Stash the specified stack size in case the VM cares */
    this.stackSize = stackSize;

    /* Set thread ID */
    tid = nextThreadID();
}

/**
 * Construct a new map including all Inheritable ThreadLocals
 * from given parent map. Called only by createInheritedMap.
 *
 * @param parentMap the map associated with parent thread.
 */
private ThreadLocalMap(ThreadLocalMap parentMap) {
    Entry[] parentTable = parentMap.table;
    int len = parentTable.length;
    setThreshold(len);
    table = new Entry[len];

    for (int j = 0; j < len; j++) {
        Entry e = parentTable[j];
        if (e != null) {
            @SuppressWarnings("unchecked")
            ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
            if (key != null) {
                Object value = key.childValue(e.value);
                Entry c = new Entry(key, value);
                int h = key.threadLocalHashCode & (len - 1);
                while (table[h] != null)
                    h = nextIndex(h, len);
                table[h] = c;
                size++;
            }
        }
    }
}