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目录
先来看基本用法
对ThreadLocal的理解
ThreadLocal 详解
正确理解 ThreadLocal
ThreadLocal 源码分析
构造方法
set 方法
ThreadLocalMap
get 方法
总结
一个类型转换的坑

class Person {ThreadLocal<Long> longLocal = new ThreadLocal<Long>(); //创建一个用于保存 Long 类型数据的 ThreadLocalThreadLocal<String> stringLocal = new ThreadLocal<String>() {//创建一个用于保存 String 类型数据的 ThreadLocal@Overrideprotected String initialValue() {return Thread.currentThread().getName(); //初始化数据第一种方式：重写 initialValue 方法(推荐方式)}};//ThreadLocal.withInitial(()-> "返回初始值"); //初始化数据第三种方式，在1.8中添加的APIpublic void setLongValue() {longLocal.set(Thread.currentThread().getId()); //初始化数据第二种方式：调用 set 方法}public long getLong() {return longLocal.get();}public String getString() {return stringLocal.get();}
}

public class Test {public static void main(String[] args) throws InterruptedException {final Person person = new Person();person.setLongValue(); //设置main线程中的对象的值System.out.println("A：" + person.getLong()); //1System.out.println("A：" + person.getString()); //mainThread thread = new Thread() {public void run() {person.setLongValue(); //设置子线程中的对象的值System.out.println("B：" + person.getLong()); //10System.out.println("B：" + person.getString()); //Thread-0};};thread.start();thread.join(); //效果等同于同步System.out.println("C：" + person.getLong()); //1System.out.println("C：" + person.getString()); //main}
}

A：1
A：main
B：10
B：Thread-0
C：1
C：main

ThreadLocal<Long> longLocal = new ThreadLocal<Long>();
protected T initialValue() //一般是用来在使用时进行重写的，它是一个延迟加载方法public void set(T value)
public T get()
public void remove()

private static final ThreadLocal threadSession = new ThreadLocal();public static Session getSession() throws InfrastructureException {Session s = (Session) threadSession.get();try {if (s == null) {s = getSessionFactory().openSession(); //创建一个sessionthreadSession.set(s);}} catch (HibernateException ex) {throw new InfrastructureException(ex);}return s;
}

ThreadLocal<Long> longLocal = new ThreadLocal<Long>();
protected T initialValue() //一般是用来在使用时进行重写的，它是一个延迟加载方法public void set(T value)
public T get()
public void remove()

private final int threadLocalHashCode = nextHashCode();  //唯一的实例变量，而且还是不可变的
private static int nextHashCode = 0;  //静态变量，表示即将分配的下一个ThreadLocal实例的threadLocalHashCode的值
private static final int HASH_INCREMENT = 0x61c88647; //常量，表示了连续分配的两个ThreadLocal实例的threadLocalHashCode值的增量

//Creates a thread local variable. @see #withInitial(java.util.function.Supplier)
public ThreadLocal() {
}

private final int threadLocalHashCode = nextHashCode();  

//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 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(); //取得当前线程ThreadLocalMap map = t.threadLocals; //返回当前线程t中的一个成员变量 threadLocalsif (map != null) map.set(this, value); //将该 ThreadLocal 实例(而不是当前线程)作为key，要保持的对象作为值else t.threadLocals = new ThreadLocalMap(this, value);
}

public class Thread implements Runnable {  //ThreadLocal values pertaining to this thread. This map is maintained by the ThreadLocal class.ThreadLocal.ThreadLocalMap threadLocals = null;
}  

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 mean that the key is no longer referenced, so the entry can be expunged擦去、移除 from table.  //Such entries are referred to as "stale陈旧的 entries" in the code that follows.static class Entry extends WeakReference<ThreadLocal<?>> {Object value;//The value associated with this ThreadLocal.Entry(ThreadLocal<?> k, Object v) {super(k);value = v;}}//...
}

//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 initialValue method.
//@return the current thread's value of this thread-local
public T get() {Thread t = Thread.currentThread();ThreadLocalMap map = t.threadLocals;if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this); //注意这里获取键值对传进去的是 ThreadLocal 实例，而不是当前线程 tif (e != null) return (T)e.value;}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 = t.threadLocals; if (map != null) map.set(this, value);else t.threadLocals = new ThreadLocalMap(this, value);return value;
}

protected T initialValue() {return null;
}

public class Test {public static void main(String[] args) {System.out.println(new Person().getLong());System.out.println(new Person().getLong2());}
}class Person {ThreadLocal<Long> longLocal = new ThreadLocal<Long>();public Long getLong() {return longLocal.get();}public long getLong2() {return longLocal.get();}
}

null
Exception in thread "main" java.lang.NullPointerExceptionat Person.getLong2(Test.java:17)at Test.main(Test.java:5)