线程的共享和协作
目录
- 并发编程的一些定义和概念
1.1、并行和并发的区别
1.2、多线程的安全注意事项 - 线程的使用
2.1、线程的启动和中止
2.2、run（）和start（）的区别
2.3、其他线程方法
2.4、synchronized内置锁和volatile关键字 - ThreadLocal
3.1、ThreadLocal的使用
3.2、ThreadLocal的与synchronized的比较
3.3、ThreadLocal的实现解析
3.4、ThreadLocal不规范使用导致的内存泄漏分析
3.5、ThreadLocal错误使用导致的线程不安全问题 - 线程的等待和通知机制
- 末尾
一些基础知识我会略过，不了解的可以看我这篇文章javaSE—多线程基础
一、并发编程的一些定义和概念
1.1、并行和并发的区别
并行：指同时运行的多个线程，宏观和微观都是同时运行
并发： 指单个CPU核心下单位时间内运行了多少个线程，宏观并行，微观串行
并行纯靠硬件技术，并发软件技术也可以出力
并发概念核心词： 单位时间内、单核
并发跟速度一样，脱离了时间限定，也就无所谓并发这概念。
多核也可以并发，上面只是为了强调并发是为了将每个CPU的运行效率最大化
1.2、多线程的安全注意事项
- 多线程的安全性：共享资源的数据一致性
- 线程死锁
- 线程太多了会将服务器资源耗尽形成死机当机。
每启动一个线程，OS将会分配 1M左右 的 栈空间 给它（还不包括代码里其他变量对象的空间）
二、线程的使用
2.1、线程的启动和中止
2.1.1 新线程的创建方式只有两种：
- Thread方式
- Runnable方式（包括了Callable）
//重看callable 和 源码
JDK英文文档——Thread类
2.1.2 线程的启动：start()
2.1.3 （重点）线程的中止：interrupt（）、stop（）
推荐使用interrupt（）
线程.stop()：将线程强制打断，不管该线程是否执行完或者释放了资源没。（不推荐使用）
线程.interrupt(): 将线程的中断标志位设为true，而不是强制打断（是线程协作式的典型）
interrupt（）的步骤：
- 在B线程中执行A.interrupt()；
- A线程中获取中断位变化：isInterrupted() 或者 Thread.interrupted();
B线程执行A线程.interrupt()，仅仅是通知A线程请它中断，A线程是否要中断，完全由A线程自己决定
isInterrupted() 与 Thread.interrupted()的区别：
- isInterrupted(）是Thread类的普通方法，仅仅返回当前线程的中断标志位的值
- Thread.interrupted(）是Thread类的 静态 方法，在返回当前线程的中断标志位的值后，还会将中断位设为false
- 在源码中isInterrupted() 和 Thread.interrupted() 的区别只有调用isInterrupted（boolean）的参数不同而已，isInterrupted()是传进去false，Thread.interrupted()是传进去true
isInterrupted()源码：
Thread.interrupted(）源码：
public class EndThread {
public static void main(String[] args) throws InterruptedException {
InterruptThread t1 = new InterruptThread("中断一号");
t1.start();
t1.sleep(50);
t1.interrupt();
// t1.stop();
}
}
class InterruptThread extends Thread{
public InterruptThread(String name) {
super(name);
}
public void run() {
// //完全不理会interrupt标志位的变化
// while(true) {
// System.out.println("我完全不理会中断位变化，不中断");
// try { sleep(1000); } catch (InterruptedException e) { }
// }
//理会interrupt标志位的变化
while(!isInterrupted()) {//返回当前线程的中断标志位的值，true中断，false不中断
// while(!Thread.interrupted()) {//返回线程的中断标志位的值,而且会修改中断位为false
System.out.println(Thread.currentThread().getName()
+"还没被中断，中断标志位："+isInterrupted());
}
//如果使用stop()，while循环后的代码都不会执行
System.out.println("执行中断后的标志位："+isInterrupted());
}
}
线程可以完全不理会interrupt标志位的变化：
isInterrupted():
Thread.interrupted():
总结：
- A线程.interrupt（）：将A线程中断标志位设为true
- isInterrupted（）：返回当前线程的中断标志位的值
- Thread.interrupted（）：返回当前线程的中断标志位的值，并将当前线程的中断位设为false
- A线程.interrupted（）：返回当前线程的中断标志位的值，并将A线程的中断位设为false（ interrupted（）是静态方法，其实三、四是一样的）
2.2、run（）和start（）的区别
start（）：启动新线程
run（）：不启动新线程，仅仅调用该线程的Runnable成员对象的run方法
Thread.start（）源码：start0()——>这里启动新线程
public synchronized void start() {
/**
* This method is not invoked for the main method thread or "system"
* group threads created/set up by the VM. Any new functionality added
* to this method in the future may have to also be added to the VM.
*
* A zero status value corresponds to state "NEW".
*/
if (threadStatus != 0)
throw new IllegalThreadStateException();
/* Notify the group that this thread is about to be started
* so that it can be added to the group's list of threads
* and the group's unstarted count can be decremented. */
group.add(this);
boolean started = false;
try {
start0();//这里启动新线程
started = true;
} finally {
try {
if (!started) {
group.threadStartFailed(this);
}
} catch (Throwable ignore) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}
private native void start0();
Thread.run（）源码：
2.3、其他线程方法
JDK中文文档——Thread类
1. currentThread()
2. sleep()：释放CPU资源，不释放锁
3. yield()：释放CPU资源，不释放锁
4. wait()：释放CPU资源，释放锁，只有wait（）可以释放锁
5. join()：释放CPU资源，释放锁，底层调用wait（），所以才能释放锁
6. stop()、suspend() 、resume()
7. run()、start()
8. get：-Id()、-Name()、-Priority()、-State()
9. set：-Name()、-Priority()、-Daemon()
10. is:-Daemon()、-Alive()
11. 中断：interrupt()
12. interrupted() (static方法)、isInterrupted()
2.4、synchronized内置锁和volatile关键字
1、synchronized
以前写的关于synchronized内置锁的文章
挺详细的，就不重复写了。
2、volatile
最低级别的同步锁，仅仅实现对共享资源的实时读取的效果，即可见性
volatile两个作用：
- 可见性：保证所有线程读取该共享变量时，都是读取到最新值
- 禁止指令重排序：保证在执行读或写volatile变量时，在该变量前所有命令都已经执行，在该变量后的所有命令都未执行。
程序猿的内心独白——这是一篇关于volatile的文章，比较详细，（后面我在自己整理一下关于volatile的文章，感觉不是很清晰）
3、整理一下：
- volatile是通过在volatile原子操作的 前 后 加Lock。即内存屏障来实现上面两个功能的
- volatile只能保证实时读取，不能保证线程安全，不能保证写操作的正确性，还有不能解决多CPU的 并行写 问题
整理这里的两个细节：
- volatile原子操作：
这是volatile的原子操作（读）
volatile int i = 0;
public static void main(String[] args) {
System.out.println(i);
}
这不是volatile的原子操作（读、写），多线程操作时就会有可能出问题
volatile int i = 0;
public static void main(String[] args) {
System.out.println(++i);
}
- volatile不能保证并行写的问题，但可以保证并行读的问题，注意是并行，不只是并发
在单cpu操作volatile变量时，JVM不会给volatile语句加特殊的LOCk，就算多线程并发也是单cpu，但是当多CPU并行操作该volatile变量时，JVM会给变量加特殊的LOCK，以保证volatile的可见性的正确性，所以，volatile可以保证并行读的正确性 - volatile能不能保证原子性？
只能保证一定程度上的原子性，即实时读取，不能保证对该变量的所有操作都是原子性。 - volatile能不能保证有序性？
只能保证一定程度上的有序性，即volatile前面都执行了，后面都没执行，但不能保证前面或者后面的代码的有序性。
林夕-长——这是关于volatile的一些细节问题总结
4、使用 volatile的应用场景：
- 对变量的写操作不依赖于当前值。（因为它不能保证并发写的正确性）
- 该变量没有包含在具有其他变量的不变式中（只保证volatile变量的读原子操作）
个人觉得禁止指令重排序是实现可见性的基础
三、ThreadLocal
ThreadLocal的中文文档
ThreadLocal，很多地方叫做线程本地变量，也有些地方叫做线程本地存储.
但其实 ，ThreadLocal 更准确的说是个 线程数据副本调用器，而不是线程本地变量/存储
详细解释留到下面的 3.3、实现解析 再说，毕竟先用用再解释原理会多点实感
3.1、ThreadLocal的使用
ThreadLocal只有四个方法，使用起来较简单，分别是：
- get（）：返回该线程数据副本值，原文：返回此线程局部变量的当前线程副本中的值。
- set（T value）： 获取该线程数据副本值，原文：将此线程局部变量的当前线程副本中的值设置为指定值。
- remove（）：移除该线程数据副本值，原文：移除此线程局部变量当前线程的值。
- initialValue（）：返回该线程数据副本值，原文：返回此线程局部变量的当前线程的“初始值”。
ThreadLocal的两种创建、get（）、set（）、remove（）：
（ThreadLocal的创建一般都是 + private static 的成员变量）
package com.Thread.ThreadLocal;
/**
*类说明：演示ThreadLocal的使用
*/
public class UseThreadLocal {
//第一种写法
private static ThreadLocal<Integer> tLocal = new ThreadLocal<>();
// private static ThreadLocal tLocal = new ThreadLocal<>();
//第二种写法,一般都是用第二种，创建时自己设初始值
// private static ThreadLocal tLocal = new ThreadLocal() { //如果要写匿名类，new后必须加Integer实际类型,不能<>。
// protected Integer initialValue() {
// return 10; //重写initialValue()，更改初始值,不改的话默认值是null
// }
// };
public static void main(String[] args) {
Runnable r1 = new Runnable() {
public void run() {
// int a = tLocal.get(); tLocal.set(++a);
tLocal.set((tLocal.get()+1));
System.out.println(Thread.currentThread().getName()+": "+tLocal.get());
}
};
for(int i=0;i<5;i++)
new Thread(r1,"T-"+i).start();
// 用第一种的Integer泛型会报错，因为它默认值是null，
// 数值为null还拿去给变量赋值，就会报空指针异常
}
}
第一种写法Integer类型结果：
第一种写法String类型结果：
第二种写法Integer类型结果：
initialValue（）：
- 供ThreadLocal需要初始化数据时使用，创建 和 remove（）后对象为null时。
- 使用：不可调用，创建时重写，起到更改默认值的作用。（像上面的第二种写法）
3.2、ThreadLocal的与synchronized的比较
ThreadLocal 和 synchronized 都用于解决多线程并发访问的问题，本质的区别在于：多线程并发访问操作的变化是否会相互影响。
- ThreadLocal：为每一个线程创建各自的线程数据副本，各自操作各自的数据，互不影响。（每个人都有自己的游戏账号，各玩各的）
- synchronized：所有线程都是用同一个数据，每个线程的操作都会影响到其他线程的操作。（所有人用同一个游戏账号，大家一起玩）
ThreadLocal利用静态内部类ThreadLocalMap提供线程副本变量。
synchronized通过锁来实现多线程并发访问同一个共享变量时的数据安全性。
3.3、ThreadLocal的实现解析
1.ThreadLocal 本身：
为什么说 ：ThreadLocal 更准确的说是个 线程数据副本调用器，而不是线程本地变量/存储？
线程本地变量/存储，实际上是ThreadLocal的静态内部类ThreadLocalMap。
你可以把ThreadLocal看成单纯的这样的一个类：集合了 多个使用 一张key—value表 的方法 的类。emmm，就一些方法而已。
ThreadLocal最特殊的地方在于：它本身不保存任何数据，是的，上面的key-value表并不是ThreadLocal对象的成员变量，value，key它都不会保存，所以才说ThreadLocal是一个 线程数据副本调用器，而不是线程副本类
2.ThreadLocal 使用的key—value表：以ThreadLocal自身作为key
ThreadLocal 的key—value表：ThreadLocalMap的成员变量Entry（ThreadLocal，Object）数组
ThreadLocalMap是ThreadLocal的静态内部类，Entry是ThreadLocalMap的静态内部类。
所以实际上是存储在ThreadLocalMap类中的Entry对象数组中
Entry：
- key：ThreadLocal类型，当前使用的ThreadLocal对象
- value：Object，你所要存储的数据对象（但是ThreadLocal用了泛型，而泛型参数都是引用对象，所以别int，要用Integer）
问题1：为什么不说ThreadLocal类是线程本地变量，或者Entry类才是线程本地变量？
实际存储是在ThreadLocalMap类中，不是ThreadLoca类，也不是Entry类。
存储地方是在ThreadLocalMap中的成员变量Entry数组中，而ThreadLocal中并没有ThreadLocalMap成员变量，Entry更是个简单的类而已，就一个骨架。
问题2：ThreadLocalMap又是在哪里呢？我们直接使用的只是ThreadLocal，又不是ThreadLocalMap。
这就是我为什么说ThreadLocal实际上是个 线程数据副本调用器 的原因
Thread线程类有一个成员变量ThreadLocalMap，JDK注明该成员变量就是用来给ThreadLocal类使用的。
我们调用ThreadLocal类实际上就是在通过ThreadLocal的方法去使用对应线程的ThreadLocalMap成员变量，所以ThreadLocal只是个调用器，而不是线程本地副本，线程本地副本实际上是线程自身的ThreadLocalMap成员变量。
3、调用流程
调用流程用get（）说一遍吧，set（）差不多就不写了
想看ThreadLocal类源码的点这里
get（）：
- 通过当前线程对象，获取当前线程的线程本地副本，直白点就是获取自身线程的线程本地副本
- 判断线程本地副本ThreadLocalMap是否为null，是null，说该线程没有任何本地副本，自然没有我们使用的ThreadLocal的对应副本，就会重新初始化副本值，并返回初始值
- 本地副本ThreadLocalMap不为空时，用我们使用的ThreadLocal对象作为key，查询是否有对应的线程副本存储值，有就返回，没有就会跟上一步一样，重新初始化副本值，并返回初始值
3.4、ThreadLocal不规范使用导致的内存泄漏分析
内存泄漏涉及到对象引用的知识，这里简单写一下
1、对象引用：
1.1 引用种类：
- 强引用：指必需的对象引用，只要强引用还在，垃圾回收器就不会回收被引用的实例 。（ 类似Object o = new Object(); 后面的new Object（）实例不会被回收，直到程序结束才会回收）
- 软引用：指有用但不是必需的对象引用。（在系统内存不足即将发生内存溢出时才会回收，如果回收后还不足就会抛出内存溢出异常）
- 弱引用：指不重要的对象引用，只要发生垃圾回收，它就会被回收
- 虚引用：也称为幽灵引用或者幻影引用，它是最弱的一种引用关系。一个对象实例是否有虚引用的存在，完全不会对其生存时间构成影响，也无法通过虚引用
来取得一个对象实例。为一个对象设置虚引用关联的唯一目的就是能在这个对象
实例被收集器回收时收到一个系统通知。
写法：
1.2 建立引用
Object o = new Object()；的三步骤：
Object o ——> 在栈中生成一个对象
new Object() ——> 在堆中生成一个对象实例
=（赋值） ——> 在 o 与 对象实例 之间 建立引用
所以Object o = null;——>在栈中生成了一个对象，却没有生成实例和建立引用
2、ThreadLocalMap的存储方式
Entry(弱引用的ThreadLocal，强引用的value）
Entry的key是弱引用，那么在程序运行时如果发生垃圾回收，那么Entry的Key就会被回收（null）
这个时候问题就来了，Entry [ ]的元素Entry类型是存储两个值的，例如Entry [0] 的 key虽然被置为null了，但value没有。
那么Entry [0] 因为value不为空，它就不会被当成null，但是Entry [0]因为key为null，实际上已经是没用的垃圾，空占空间（value的大小）。
这就是ThreadLocal的内存泄漏问题的根源
解决办法；remove（）——>及时清除不用的线程本地副本，用完就删
3.5、ThreadLocal错误使用导致的线程不安全问题
使用ThreadLocal存储数据到线程本地变量时，存储的数据不能是static修饰的
原因：static变量是唯一引用实例，就是使用了static后，不管你怎么写，你用的都会是同一个实例
而ThreadLocal的本质是通过ThreadLocalMap实现不同的线程本地副本，自然会出现线程不安全问题
加了static后就跟没写ThreadLocal一样，就不演示了
四、线程的等待和通知机制
wait（）会释放锁
- 锁. wait（）：不限时间的等待，就一直等待
- 锁. wait（long a）：最多等待a毫秒，时间到了会自动醒来
- 锁. wait（long a，int b）：最多等待a毫秒 + b纳秒，时间到了会自动醒来
- 锁. notify（）：随机唤醒，少用
- 锁. notifyAll（）：全唤醒，多用
(返回 调用流程)ThreadLocal全文源码：
* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.lang;
import java.lang.ref.*;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;
/**
* This class provides thread-local variables. These variables differ from
* their normal counterparts in that each thread that accesses one (via its
* {@code get} or {@code set} method) has its own, independently initialized
* copy of the variable. {@code ThreadLocal} instances are typically private
* static fields in classes that wish to associate state with a thread (e.g.,
* a user ID or Transaction ID).
*
* For example, the class below generates unique identifiers local to each
* thread.
* A thread's id is assigned the first time it invokes {@code ThreadId.get()}
* and remains unchanged on subsequent calls.
*
* import java.util.concurrent.atomic.AtomicInteger;
*
* public class ThreadId {
* // Atomic integer containing the next thread ID to be assigned
* private static final AtomicInteger nextId = new AtomicInteger(0);
*
* // Thread local variable containing each thread's ID
* private static final ThreadLocal<Integer> threadId =
* new ThreadLocal<Integer>() {
* @Override protected Integer initialValue() {
* return nextId.getAndIncrement();
* }
* };
*
* // Returns the current thread's unique ID, assigning it if necessary
* public static int get() {
* return threadId.get();
* }
* }
*
* Each thread holds an implicit reference to its copy of a thread-local
* variable as long as the thread is alive and the {@code ThreadLocal}
* instance is accessible; after a thread goes away, all of its copies of
* thread-local instances are subject to garbage collection (unless other
* references to these copies exist).
*
* @author Josh Bloch and Doug Lea
* @since 1.2
*/
public class ThreadLocal<T> {
/**
* ThreadLocals rely on per-thread linear-probe hash maps attached
* to each thread (Thread.threadLocals and
* inheritableThreadLocals). The ThreadLocal objects act as keys,
* searched via threadLocalHashCode. This is a custom hash code
* (useful only within ThreadLocalMaps) that eliminates collisions
* in the common case where consecutively constructed ThreadLocals
* are used by the same threads, while remaining well-behaved in
* less common cases.
*/
private final int threadLocalHashCode = nextHashCode();
/**
* The next hash code to be given out. Updated atomically. Starts at
* zero.
*/
private static AtomicInteger nextHashCode =
new AtomicInteger();
/**
* The difference between successively generated hash codes - turns
* implicit sequential thread-local IDs into near-optimally spread
* multiplicative hash values for power-of-two-sized tables.
*/
private static final int HASH_INCREMENT = 0x61c88647;
/**
* Returns the next hash code.
*/
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
/**
* 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}.
*
* 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;
}
/**
* Creates a thread local variable. The initial value of the variable is
* determined by invoking the {@code get} method on the {@code Supplier}.
*
* @param the type of the thread local's value
* @param supplier the supplier to be used to determine the initial value
* @return a new thread local variable
* @throws NullPointerException if the specified supplier is null
* @since 1.8
*/
public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
return new SuppliedThreadLocal<>(supplier);
}
/**
* Creates a thread local variable.
* @see #withInitial(java.util.function.Supplier)
*/
public ThreadLocal() {
}
/**
* 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();
}
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
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;
}
/**
* 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);
}
/**
* 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;
}
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
/**
* Factory method to create map of inherited thread locals.
* Designed to be called only from Thread constructor.
*
* @param parentMap the map associated with parent thread
* @return a map containing the parent's inheritable bindings
*/
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
/**
* Method childValue is visibly defined in subclass
* InheritableThreadLocal, but is internally defined here for the
* sake of providing createInheritedMap factory method without
* needing to subclass the map class in InheritableThreadLocal.
* This technique is preferable to the alternative of embedding
* instanceof tests in methods.
*/
T childValue(T parentValue) {
throw new UnsupportedOperationException();
}
/**
* An extension of ThreadLocal that obtains its initial value from
* the specified {@code Supplier}.
*/
static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
private final Supplier<? extends T> supplier;
SuppliedThreadLocal(Supplier<? extends T> supplier) {
this.supplier = Objects.requireNonNull(supplier);
}
@Override
protected T initialValue() {
return supplier.get();
}
}
/**
* ThreadLocalMap is a customized hash map suitable only for
* maintaining thread local values. No operations are exported
* outside of the ThreadLocal class. The class is package private to
* allow declaration of fields in class Thread. To help deal with
* very large and long-lived usages, the hash table entries use
* WeakReferences for keys. However, since reference queues are not
* used, stale entries are guaranteed to be removed only when
* the table starts running out of space.
*/
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;
}
}
/**
* The initial capacity -- MUST be a power of two.
*/
private static final int INITIAL_CAPACITY = 16;
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
/**
* The number of entries in the table.
*/
private int size = 0;
/**
* The next size value at which to resize.
*/
private int threshold; // Default to 0
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
/**
* Increment i modulo len.
*/
private static int nextIndex(int i, int len) {
return ((i + 1 < len) ? i + 1 : 0);
}
/**
* Decrement i modulo len.
*/
private static int prevIndex(int i, int len) {
return ((i - 1 >= 0) ? i - 1 : len - 1);
}
/**
* Construct a new map initially containing (firstKey, firstValue).
* ThreadLocalMaps are constructed lazily, so we only create
* one when we have at least one entry to put in it.
*/
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
/**
* 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++;
}
}
}
}
/**
* Get the entry associated with key. This method
* itself handles only the fast path: a direct hit of existing
* key. It otherwise relays to getEntryAfterMiss. This is
* designed to maximize performance for direct hits, in part
* by making this method readily inlinable.
*
* @param key the thread local object
* @return the entry associated with key, or null if no such
*/
private Entry getEntry(ThreadLocal<?> key) {
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
/**
* Version of getEntry method for use when key is not found in
* its direct hash slot.
*
* @param key the thread local object
* @param i the table index for key's hash code
* @param e the entry at table[i]
* @return the entry associated with key, or null if no such
*/
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
ThreadLocal<?> k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
/**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void set(ThreadLocal<?> key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
/**
* Remove the entry for key.
*/
private void remove(ThreadLocal<?> key) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
if (e.get() == key) {
e.clear();
expungeStaleEntry(i);
return;
}
}
}
/**
* Replace a stale entry encountered during a set operation
* with an entry for the specified key. The value passed in
* the value parameter is stored in the entry, whether or not
* an entry already exists for the specified key.
*
* As a side effect, this method expunges all stale entries in the
* "run" containing the stale entry. (A run is a sequence of entries
* between two null slots.)
*
* @param key the key
* @param value the value to be associated with key
* @param staleSlot index of the first stale entry encountered while
* searching for key.
*/
private void replaceStaleEntry(ThreadLocal<?> key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
// Back up to check for prior stale entry in current run.
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (i.e., whenever the collector runs).
int slotToExpunge = staleSlot;
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
// Find either the key or trailing null slot of run, whichever
// occurs first
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
// If we find key, then we need to swap it
// with the stale entry to maintain hash table order.
// The newly stale slot, or any other stale slot
// encountered above it, can then be sent to expungeStaleEntry
// to remove or rehash all of the other entries in run.
if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
// If we didn't find stale entry on backward scan, the
// first stale entry seen while scanning for key is the
// first still present in the run.
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
// If key not found, put new entry in stale slot
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
/**
* Expunge a stale entry by rehashing any possibly colliding entries
* lying between staleSlot and the next null slot. This also expunges
* any other stale entries encountered before the trailing null. See
* Knuth, Section 6.4
*
* @param staleSlot index of slot known to have null key
* @return the index of the next null slot after staleSlot
* (all between staleSlot and this slot will have been checked
* for expunging).
*/
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
int h = k.threadLocalHashCode & (len - 1);
if (h != i) {
tab[i] = null;
// Unlike Knuth 6.4 Algorithm R, we must scan until
// null because multiple entries could have been stale.
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
/**
* Heuristically scan some cells looking for stale entries.
* This is invoked when either a new element is added, or
* another stale one has been expunged. It performs a
* logarithmic number of scans, as a balance between no
* scanning (fast but retains garbage) and a number of scans
* proportional to number of elements, that would find all
* garbage but would cause some insertions to take O(n) time.
*
* @param i a position known NOT to hold a stale entry. The
* scan starts at the element after i.
*
* @param n scan control: {@code log2(n)} cells are scanned,
* unless a stale entry is found, in which case
* {@code log2(table.length)-1} additional cells are scanned.
* When called from insertions, this parameter is the number
* of elements, but when from replaceStaleEntry, it is the
* table length. (Note: all this could be changed to be either
* more or less aggressive by weighting n instead of just
* using straight log n. But this version is simple, fast, and
* seems to work well.)
*
* @return true if any stale entries have been removed.
*/
private boolean cleanSomeSlots(int i, int n) {
boolean removed = false;
Entry[] tab = table;
int len = tab.length;
do {
i = nextIndex(i, len);
Entry e = tab[i];
if (e != null && e.get() == null) {
n = len;
removed = true;
i = expungeStaleEntry(i);
}
} while ( (n >>>= 1) != 0);
return removed;
}
/**
* Re-pack and/or re-size the table. First scan the entire
* table removing stale entries. If this doesn't sufficiently
* shrink the size of the table, double the table size.
*/
private void rehash() {
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
}
/**
* Double the capacity of the table.
*/
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0;
for (int j = 0; j < oldLen; ++j) {
Entry e = oldTab[j];
if (e != null) {
ThreadLocal<?> k = e.get();
if (k == null) {
e.value = null; // Help the GC
} else {
int h = k.threadLocalHashCode & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}
setThreshold(newLen);
size = count;
table = newTab;
}
/**
* Expunge all stale entries in the table.
*/
private void expungeStaleEntries() {
Entry[] tab = table;
int len = tab.length;
for (int j = 0; j < len; j++) {
Entry e = tab[j];
if (e != null && e.get() == null)
expungeStaleEntry(j);
}
}
}
}
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