一、线程
我们都知道，进程是系统运行程序的基本单位，打开电脑的任务管理器，就可以看到一个个运行中的进程。但是仔细观察可以发现每个进程之下，还有更多的线程。
线程是比进程更小、更轻量级的执行单位，每个进程都拥有自己的一块内存空间和变量资源等，然而同一个进程下的多个线程则共享数据和资源，所以不管线程的创建和销毁工作，还是在线程之间切换工作，都要比进程更加轻量级、消耗系统资源更少。
二、Java多线程编程
在没有接触多线程之前，我们编写的程序都只有一个作为程序入口的main函数，其实这正是一个线程Main，当然这是单线程下的编程。
1、创建多线程
初学Java多线程编程的时候，需要掌握两种创建多线程的方法：
- 声明一个Thread类的子类，子类中重写Thread类的run方法。
- 声明一个实现Runnable接口的类，类中实现run方法。
更推荐使用第二种方式创建多线程，Thread类本身也实现了Runnable接口。
public class ThreadTest {
public static void main(String[] args) {
MyThread t1 = new MyThread();
MyThreadRunnable target = new MyThreadRunnable();
Thread t2 = new Thread(target);
//启动线程1
t1.start();
//启动线程2
t2.start();
}
}
//创建线程方式一
class MyThread extends Thread {
@Override
public void run() {
for (int i = 0; i < 1000; i++) {
System.out.println("方式一-->" + i);
}
}
}
//创建线程方式二
class MyThreadRunnable implements Runnable {
@Override
public void run() {
for (int i = 0; i < 1000; i++) {
System.out.println("方式二-->" + i);
}
}
}
2、线程状态
Java中线程可以有如下6中状态：
- NEW 新创建
- RUNNABLE 可运行
- BLOCKED 阻塞
- WAITING 等待
- TIMED WAITING 计时等待
- TERMINATED 终止
线程之间的状态转换如下图（图源于《Java并发编程艺术》）：
Java 线程状态变迁
三、Thread类详解
Thread类是java.lang包下的类，是多线程经常需要使用的类。因为笔者之前看过一些关于多线程的书和资料，包括《Java核心技术》的并发一章和其他文章等，但是始终无法理解其中原理，总是容易遗忘。索性这次结合着源码进行学习，笔者通过阅读Thread类的源码（笔者阅读的是JDK8的源码），并对常用的方法作出了自己的注释，确实学习效果要比之前只看资料来得扎实。
其中，笔者对一些初学者容易产生疑惑的地方都作出了注释，比如join方法的使用，线程中断等。
笔者建议大家可以将下面的代码复制进自己的一个单独的类文件，然后对比笔者的注释（这里可以通过Ctrl+f进行搜索，快速定位），结合自己阅读源码，去开始探索多线程领域。
/*
* Copyright (c) 1994, 2016, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.lang;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.PrivilegedAction;
import java.util.Map;
import java.util.HashMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.LockSupport;
import sun.nio.ch.Interruptible;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.security.util.SecurityConstants;
/*
在某个线程中创建一个新线程，新线程的优先级和创建新线程的线程一致，如果创建线程是守护线程的话，则新线程
是守护线程。
当Java虚拟机启动时，通常有一个非守护线程（即main线程，调用指定类的main方法），Java虚拟机会继续执行这些线程
直到发生如下情形之一：
· 调用了类Runtime的exit()方法，并且安全管理器允许发生退出操作。
· 所有非守护线程均已死亡，可以是通过执行完run方法后返回，也可以是执行run方法时抛出了异常。
创建新的执行线程的两种方法：
· 声明一个Thread类的子类，子类中重写Thread类的run方法。
· 声明一个实现Runnable接口的类，类中实现run方法。
每个线程都有一个名字以供识别。线程名字可以相同。如果创建线程时未指定名字，会分配一个新名字。
除非特殊说明，否则将null参数传递给Thread类的构造器或者方法中会导致NullPointerException。
*/
public
class Thread implements Runnable {
/* Make sure registerNatives is the first thing does. */
private static native void registerNatives();
static {
registerNatives();
}
private volatile String name; //线程名称
private int priority; //线程优先级
private Thread threadQ;
private long eetop;
/* Whether or not to single_step this thread. */
private boolean single_step;
/* Whether or not the thread is a daemon thread. */
private boolean daemon = false; //是否守护线程标识，初始false
/* JVM state */
private boolean stillborn = false;
/* What will be run. */
private Runnable target; //线程执行部分
/* The group of this thread */
private ThreadGroup group; //线程的线程组
/* The context ClassLoader for this thread */
private ClassLoader contextClassLoader; //线程的上下文类加载器
/* The inherited AccessControlContext of this thread */
private AccessControlContext inheritedAccessControlContext;
/* For autonumbering anonymous threads. */
private static int threadInitNumber; //静态域，用于分配线程名称
private static synchronized int nextThreadNum() {
return threadInitNumber++;
}
ThreadLocal.ThreadLocalMap threadLocals = null;
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
private long stackSize;
/*
* JVM-private state that persists after native thread termination.
*/
private long nativeParkEventPointer;
private long tid; //线程ID
private static long threadSeqNumber; //静态域，用于分配线程ID
private volatile int threadStatus = 0; //线程状态，初始值表示新创建还未启动
//静态同步方法，用于返回分配给线程的ID
private static synchronized long nextThreadID() {
return ++threadSeqNumber;
}
volatile Object parkBlocker;
private volatile Interruptible blocker;
private final Object blockerLock = new Object(); //锁对象
void blockedOn(Interruptible b) {
synchronized (blockerLock) {
blocker = b;
}
}
public final static int MIN_PRIORITY = 1; //最小线程优先级
public final static int NORM_PRIORITY = 5; //正常线程优先级
public final static int MAX_PRIORITY = 10; //最大线程优先级
//返回当前的执行线程对象
public static native Thread currentThread();
//静态方法，导致当前线程处于让步状态，表示当前线程愿意让出CPU资源使得其他线程执行
public static native void yield();
/*
静态方法，使当前执行的线程休眠给定的毫秒数，该线程不会放弃所获取的所有锁
如果某个线程正在sleep的过程中被别的线程打断，则该线程将抛出InterruptedException异常，并且该线程的中断位会被清除
*/
public static native void sleep(long millis) throws InterruptedException;
public static void sleep(long millis, int nanos)
throws InterruptedException {
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}
if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
millis++;
}
sleep(millis);
}
private void init(ThreadGroup g, Runnable target, String name,
long stackSize) {
init(g, target, name, stackSize, null, true);
}
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(); //将创建新线程的线程作为父线程
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(); //和父线程同一个线程组
}
}
/* 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(); //新线程默认与父线程的守护属性保持一致
this.priority = parent.getPriority(); //新线程默认与父线程的优先级保持一致
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);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID(); //设置线程ID
}
@Override
protected Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}
//可以发现：线程名称是采用 "Thread-" + 数字 的方式
public Thread() {
init(null, null, "Thread-" + nextThreadNum(), 0);
}
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
Thread(Runnable target, AccessControlContext acc) {
init(null, target, "Thread-" + nextThreadNum(), 0, acc, false);
}
public Thread(ThreadGroup group, Runnable target) {
init(group, target, "Thread-" + nextThreadNum(), 0);
}
public Thread(String name) {
init(null, null, name, 0);
}
public Thread(ThreadGroup group, String name) {
init(group, null, name, 0);
}
public Thread(Runnable target, String name) {
init(null, target, name, 0);
}
public Thread(ThreadGroup group, Runnable target, String name) {
init(group, target, name, 0);
}
public Thread(ThreadGroup group, Runnable target, String name,
long stackSize) {
init(group, target, name, stackSize);
}
//同步方法
public synchronized void start() {
//start()方法只能调用一次，否则抛出该异常
if (threadStatus != 0)
throw new IllegalThreadStateException();
group.add(this);
boolean started = false;
try {
start0();
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();
@Override
public void run() {
if (target != null) {
target.run();
}
}
private void exit() {
if (group != null) {
group.threadTerminated(this);
group = null;
}
/* Aggressively null out all reference fields: see bug 4006245 */
target = null;
/* Speed the release of some of these resources */
threadLocals = null;
inheritableThreadLocals = null;
inheritedAccessControlContext = null;
blocker = null;
uncaughtExceptionHandler = null;
}
@Deprecated
public final void stop() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
checkAccess();
if (this != Thread.currentThread()) {
security.checkPermission(SecurityConstants.STOP_THREAD_PERMISSION);
}
}
// A zero status value corresponds to "NEW", it can't change to
// not-NEW because we hold the lock.
if (threadStatus != 0) {
resume(); // Wake up thread if it was suspended; no-op otherwise
}
// The VM can handle all thread states
stop0(new ThreadDeath());
}
@Deprecated
public final synchronized void stop(Throwable obj) {
throw new UnsupportedOperationException();
}
/*
中断此线程（即中断状态被置为true），除非该线程正在中断自己，否则总是允许的。
如果该线程因为调用了Object类的wait()方法或者Thread类的join()、sleep()方法处于阻塞状态，则该线程
的中断状态会被清除并且产生InterruptedException.
*/
public void interrupt() {
if (this != Thread.currentThread())
checkAccess();
synchronized (blockerLock) {
Interruptible b = blocker;
if (b != null) {
interrupt0(); // Just to set the interrupt flag
b.interrupt(this);
return;
}
}
interrupt0();
interrupt0();
}
/*
这是静态方法，检测当前线程是否处于中断状态，同时线程的中断状态会被该方法清除（即重置为false）。
*/
public static boolean interrupted() {
return currentThread().isInterrupted(true);
}
/*
这是实例方法，检测该线程（调用该方法的线程实例）是否处于中断状态，调用该方法不会改变线程的中断状态。
*/
public boolean isInterrupted() {
return isInterrupted(false);
}
private native boolean isInterrupted(boolean ClearInterrupted);
@Deprecated
public void destroy() {
throw new NoSuchMethodError();
}
//检测该线程是否处于活动状态，活动状态是指该线程被启动（调用start()方法）了并且没有死亡
public final native boolean isAlive();
@Deprecated
public final void suspend() {
checkAccess();
suspend0();
}
@Deprecated
public final void resume() {
checkAccess();
resume0();
}
//设置线程的优先级
public final void setPriority(int newPriority) {
ThreadGroup g;
checkAccess();
if (newPriority > MAX_PRIORITY || newPriority < MIN_PRIORITY) {//参数范围检验
throw new IllegalArgumentException();
}
if((g = getThreadGroup()) != null) {
if (newPriority > g.getMaxPriority()) {
newPriority = g.getMaxPriority();
}
setPriority0(priority = newPriority);
}
}
public final int getPriority() {
return priority;
}
public final synchronized void setName(String name) {
checkAccess();
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name;
if (threadStatus != 0) {
setNativeName(name);
}
}
public final String getName() {
return name;
}
public final ThreadGroup getThreadGroup() {
return group;
}
public static int activeCount() {
return currentThread().getThreadGroup().activeCount();
}
public static int enumerate(Thread tarray[]) {
return currentThread().getThreadGroup().enumerate(tarray);
}
@Deprecated
public native int countStackFrames();
public final synchronized void join(long millis)
throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (millis == 0) {
while (isAlive()) {
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
public final synchronized void join(long millis, int nanos)
throws InterruptedException {
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}
if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
millis++;
}
join(millis);
}
/*
等待指定的线程终止。
这里不少初学者都会有个疑惑：到底是谁等待谁终止？
回答：假如在A线程中调用了B线程的join()方法（即B.join()），则表示A线程此时进入等待状态，
会等待B线程终止才会继续执行。
*/
public final void join() throws InterruptedException {
join(0);
}
public static void dumpStack() {
new Exception("Stack trace").printStackTrace();
}
/*
设置此线程是否为守护线程，true表示守护线程，false表示用户进程。这个方法必须在调用该线程的start()
方法之前调用，否则会抛出java.lang.IllegalThreadStateException
当所有运行的线程都是守护线程时，Java虚拟机将会退出。
*/
public final void setDaemon(boolean on) {
checkAccess();
if (isAlive()) {
throw new IllegalThreadStateException();
}
daemon = on;
}
//检测该线程是否是守护线程
public final boolean isDaemon() {
return daemon;
}
public final void checkAccess() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkAccess(this);
}
}
public String toString() {
ThreadGroup group = getThreadGroup();
if (group != null) {
return "Thread[" + getName() + "," + getPriority() + "," +
group.getName() + "]";
} else {
return "Thread[" + getName() + "," + getPriority() + "," +
"" + "]";
}
}
@CallerSensitive
public ClassLoader getContextClassLoader() {
if (contextClassLoader == null)
return null;
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
ClassLoader.checkClassLoaderPermission(contextClassLoader,
Reflection.getCallerClass());
}
return contextClassLoader;
}
public void setContextClassLoader(ClassLoader cl) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
sm.checkPermission(new RuntimePermission("setContextClassLoader"));
}
contextClassLoader = cl;
}
public static native boolean holdsLock(Object obj);
private static final StackTraceElement[] EMPTY_STACK_TRACE
= new StackTraceElement[0];
public StackTraceElement[] getStackTrace() {
if (this != Thread.currentThread()) {
// check for getStackTrace permission
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkPermission(
SecurityConstants.GET_STACK_TRACE_PERMISSION);
}
// optimization so we do not call into the vm for threads that
// have not yet started or have terminated
if (!isAlive()) {
return EMPTY_STACK_TRACE;
}
StackTraceElement[][] stackTraceArray = dumpThreads(new Thread[] {this});
StackTraceElement[] stackTrace = stackTraceArray[0];
// a thread that was alive during the previous isAlive call may have
// since terminated, therefore not having a stacktrace.
if (stackTrace == null) {
stackTrace = EMPTY_STACK_TRACE;
}
return stackTrace;
} else {
// Don't need JVM help for current thread
return (new Exception()).getStackTrace();
}
}
public static Map getAllStackTraces() {
// check for getStackTrace permission
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkPermission(
SecurityConstants.GET_STACK_TRACE_PERMISSION);
security.checkPermission(
SecurityConstants.MODIFY_THREADGROUP_PERMISSION);
}
// Get a snapshot of the list of all threads
Thread[] threads = getThreads();
StackTraceElement[][] traces = dumpThreads(threads);
Map m = new HashMap<>(threads.length);
for (int i = 0; i < threads.length; i++) {
StackTraceElement[] stackTrace = traces[i];
if (stackTrace != null) {
m.put(threads[i], stackTrace);
}
// else terminated so we don't put it in the map
}
return m;
}
private static final RuntimePermission SUBCLASS_IMPLEMENTATION_PERMISSION =
new RuntimePermission("enableContextClassLoaderOverride");
private static class Caches {
/** cache of subclass security audit results */
static final ConcurrentMap subclassAudits =
new ConcurrentHashMap<>();
/** queue for WeakReferences to audited subclasses */
static final ReferenceQueue> subclassAuditsQueue =
new ReferenceQueue<>();
}
private static boolean isCCLOverridden(Class cl) {
if (cl == Thread.class)
return false;
processQueue(Caches.subclassAuditsQueue, Caches.subclassAudits);
WeakClassKey key = new WeakClassKey(cl, Caches.subclassAuditsQueue);
Boolean result = Caches.subclassAudits.get(key);
if (result == null) {
result = Boolean.valueOf(auditSubclass(cl));
Caches.subclassAudits.putIfAbsent(key, result);
}
return result.booleanValue();
}
private static boolean auditSubclass(final Class subcl) {
Boolean result = AccessController.doPrivileged(
new PrivilegedAction() {
public Boolean run() {
for (Class cl = subcl;
cl != Thread.class;
cl = cl.getSuperclass())
{
try {
cl.getDeclaredMethod("getContextClassLoader", new Class[0]);
return Boolean.TRUE;
} catch (NoSuchMethodException ex) {
}
try {
Class[] params = {ClassLoader.class};
cl.getDeclaredMethod("setContextClassLoader", params);
return Boolean.TRUE;
} catch (NoSuchMethodException ex) {
}
}
return Boolean.FALSE;
}
}
);
return result.booleanValue();
}
private native static StackTraceElement[][] dumpThreads(Thread[] threads);
private native static Thread[] getThreads();
public long getId() {
return tid;
}
/*
线程的六种状态：
NEW 新创建
RUNNABLE 可运行
BLOCKED 阻塞
WAITING 等待
TIMED_WAITING 计时等待
TERMINATED 终止
*/
public enum State {
//实例化一个线程后还没有调用其start()方法，处于新创建状态
NEW,
//处于可运行状态的线程可能正在运行也可能未运行（等待操作系统调度）
RUNNABLE,
//尝试获取锁但是未成功，等待锁
BLOCKED,
/*
一个线程调用了下面三个方法会进入等待状态：
Object.wait()
Thread.join()
LockSupport.park()
*/
WAITING,
/*
一个线程调用了下面的方法会进入超时等待状态：
Thread.sleep()
带参数的Object.wait()
带参数的Thread.join()
LockSupport.parkNanos()
LockSupport.parkUntil()
*/
TIMED_WAITING,
//run方法执行结束返回后或者run方法抛出未捕获的异常会导致线程终止
TERMINATED;
}
/*
返回该线程的状态
*/
public State getState() {
// get current thread state
return sun.misc.VM.toThreadState(threadStatus);
}
// Added in JSR-166
@FunctionalInterface
public interface UncaughtExceptionHandler {
void uncaughtException(Thread t, Throwable e);
}
// null unless explicitly set
private volatile UncaughtExceptionHandler uncaughtExceptionHandler;
// null unless explicitly set
private static volatile UncaughtExceptionHandler defaultUncaughtExceptionHandler;
public static void setDefaultUncaughtExceptionHandler(UncaughtExceptionHandler eh) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
sm.checkPermission(
new RuntimePermission("setDefaultUncaughtExceptionHandler")
);
}
defaultUncaughtExceptionHandler = eh;
}
public static UncaughtExceptionHandler getDefaultUncaughtExceptionHandler(){
return defaultUncaughtExceptionHandler;
}
public UncaughtExceptionHandler getUncaughtExceptionHandler() {
return uncaughtExceptionHandler != null ?
uncaughtExceptionHandler : group;
}
public void setUncaughtExceptionHandler(UncaughtExceptionHandler eh) {
checkAccess();
uncaughtExceptionHandler = eh;
}
private void dispatchUncaughtException(Throwable e) {
getUncaughtExceptionHandler().uncaughtException(this, e);
}
static void processQueue(ReferenceQueue> queue,
ConcurrentMap>, ?> map)
{
Reference> ref;
while((ref = queue.poll()) != null) {
map.remove(ref);
}
}
static class WeakClassKey extends WeakReference> {
private final int hash;
WeakClassKey(Class cl, ReferenceQueue> refQueue) {
super(cl, refQueue);
hash = System.identityHashCode(cl);
}
@Override
public int hashCode() {
return hash;
}
@Override
public boolean equals(Object obj) {
if (obj == this)
return true;
if (obj instanceof WeakClassKey) {
Object referent = get();
return (referent != null) &&
(referent == ((WeakClassKey) obj).get());
} else {
return false;
}
}
}
@sun.misc.Contended("tlr")
long threadLocalRandomSeed;
@sun.misc.Contended("tlr")
int threadLocalRandomProbe;
@sun.misc.Contended("tlr")
int threadLocalRandomSecondarySeed;
/* Some private helper methods */
private native void setPriority0(int newPriority);
private native void stop0(Object o);
private native void suspend0();
private native void resume0();
private native void interrupt0();
private native void setNativeName(String name);
}