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大家好，我是呼噜噜，最近一直在梳理Java并发，但内容杂且偏晦涩，今天我们一起来聊聊Java 线程的状态及转换 先来夯实一下基础，万丈高楼平地起，路还是得慢慢走。

(资料图片)

我们先来看下Java线程的生命周期图：

上图也是本文的大纲，我们下面依次聊聊java各个线程状态及其他们的转换。

线程初始状态(NEW): 当前线程处于线程被创建出来但没有被调用start()

在Java线程的时间中，关于线程的一切的起点是从Thread 类的对象的创建开始，一般实现Runnable接口 或者 继承Thread类的类，实例化一个对象出来，线程就进入了初始状态

Thread thread = new Thread()

由于线程在我们操作系统中也是非常宝贵的资源，在实际开发中，我们常常用线程池来重复利用现有的线程来执行任务，避免多次创建和销毁线程，从而降低创建和销毁线程过程中的代价。Java 给我们提供了 Executor 接口来使用线程池，查看其JDK1.8源码，发现其内部封装了Thread t = new Thread()

public class Executors {    ...  static class DefaultThreadFactory implements ThreadFactory {        private static final AtomicInteger poolNumber = new AtomicInteger(1);        private final ThreadGroup group;        private final AtomicInteger threadNumber = new AtomicInteger(1);        private final String namePrefix;        ...        public Thread newThread(Runnable r) {            Thread t = new Thread(group, r,                                  namePrefix + threadNumber.getAndIncrement(),                                  0);            if (t.isDaemon())                t.setDaemon(false);            if (t.getPriority() != Thread.NORM_PRIORITY)                t.setPriority(Thread.NORM_PRIORITY);            return t;        }    }    ...}

在thread类源码中，我们还能发现线程状态的枚举类State：

public enum State {        /**         * Thread state for a thread which has not yet started.         */        NEW,        RUNNABLE,        BLOCKED,        WAITING,        TIMED_WAITING,        /**         * Thread state for a terminated thread.         * The thread has completed execution.         */        TERMINATED;    }

所谓线程的状态，在java源码中都是通过threadStatus的值来表示的

/* Java thread status for tools,     * initialized to indicate thread "not yet started"     */    private volatile int threadStatus = 0;

State和 threadStatus通过toThreadState方法映射转换

public State getState() {        // get current thread state        return sun.misc.VM.toThreadState(threadStatus);    }//--- --- ---    public static State toThreadState(int var0) {        if ((var0 & 4) != 0) {            return State.RUNNABLE;        } else if ((var0 & 1024) != 0) {            return State.BLOCKED;        } else if ((var0 & 16) != 0) {            return State.WAITING;        } else if ((var0 & 32) != 0) {            return State.TIMED_WAITING;        } else if ((var0 & 2) != 0) {            return State.TERMINATED;        } else {            return (var0 & 1) == 0 ? State.NEW : State.RUNNABLE;        }    }

到这里我们就可以发现，Thread t = new Thread()在Java中只是设置了线程的状态，操作系统中并没有的实际线程的创建

线程运行状态(RUNNABLE)，线程被调用了start()等待运行的状态

在Linux操作系统层面，包含Running和 Ready 状态。其中Ready状态是等待 CPU 时间片。现今主流的JVM，比如hotspot虚拟机都是把Java 线程，映射到操作系统OS底层的线程上，把调度委托给了操作系统。而操作系统比如Linux,它是多任务操作系统，充分利用CPU的高性能，将CPU的时间分片，让单个CPU实现"同时执行"多任务的效果。

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Linux的任务调度又采用抢占式轮转调度，我们不考虑特权进程的话，OS会选择在CPU上占用的时间最少进程,优先在cpu上分配资源，其对应的线程去执行任务，尽可能地维护任务调度公平。Running和 Ready 状态的线程在CPU中切换状态非常短暂。大概只有 0.01 秒这一量级，区分开来意义不大，java将这2个状态统一用RUNNABLE来表示

我们接下来看看为什么说执行thread.start()后，线程的才"真正的创建"

public class ThreadTest {    /**     * 继承Thread类     */    public static class MyThread extends Thread {        @Override        public void run() {            System.out.println("This is child thread");        }    }    public static void main(String[] args) {        MyThread thread = new MyThread();        thread.start();    }}

其中thread.start()方法的源码中，会去调用start0()方法，而start0()是private native void start0();JVM调用Native方法的话，会进入到不受JVM控制的世界里

在Thread类实例化的同时，会首先调用registerNatives方法，注册本地Native方法,动态绑定JVM方法

private static native void registerNatives();    static {        registerNatives();    }

在Thread类中通过registerNatives将指定的本地方法绑定到指定函数，比如start0本地方法绑定到JVM_StartThread函数：

...static JNINativeMethod methods[] = {    {"start0",           "()V",        (void *)&JVM_StartThread},    {"stop0",            "(" OBJ ")V", (void *)&JVM_StopThread},    {"isAlive",          "()Z",        (void *)&JVM_IsThreadAlive},    ...

源码见：http://hg.openjdk.java.net/jdk8u/jdk8u60/jdk/file/935758609767/src/share/native/java/lang/Thread.c

JVM_StartThread 是JVM层函数,抛去各种情况的处理，主要是通过new JavaThread(&thread_entry, sz)来创建JVM线程对象

JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))  JVMWrapper("JVM_StartThread");  JavaThread *native_thread = NULL;//表示是否有异常，当抛出异常时需要获取Heap_lock。  bool throw_illegal_thread_state = false;  // 在发布jvmti事件之前，必须释放Threads_lock  // in Thread::start.  {    // 获取 Threads_lock锁    MutexLocker mu(Threads_lock);    if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) {      throw_illegal_thread_state = true;    } else {      // We could also check the stillborn flag to see if this thread was already stopped, but      // for historical reasons we let the thread detect that itself when it starts running      jlong size =             java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread));              // 创建JVM线程(用JavaThread对象表示)      size_t sz = size > 0 ? (size_t) size : 0;      native_thread = new JavaThread(&thread_entry, sz);      ...    }  }  ...  Thread::start(native_thread);//启动内核线程JVM_END

源码见：https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/69087d08d473/src/share/vm/prims/jvm.cpp

我们再来看看JavaThread的实现，发现内部通过 os::create_thread(this, thr_type, stack_sz);来调用不同操作系统的创建线程方法创建线程。

JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz) :  Thread()#if INCLUDE_ALL_GCS  , _satb_mark_queue(&_satb_mark_queue_set),  _dirty_card_queue(&_dirty_card_queue_set)#endif // INCLUDE_ALL_GCS{  if (TraceThreadEvents) {    tty->print_cr("creating thread %p", this);  }  initialize();  _jni_attach_state = _not_attaching_via_jni;  set_entry_point(entry_point);  // Create the native thread itself.  // %note runtime_23  os::ThreadType thr_type = os::java_thread;  thr_type = entry_point == &compiler_thread_entry ? os::compiler_thread :                                                     os::java_thread;  os::create_thread(this, thr_type, stack_sz);//调用不同操作系统的创建线程方法创建线程}

源码见：https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/69087d08d473/src/share/vm/runtime/thread.cpp

我们都知道Java是跨平台的，但是native各种方法底层c/c++代码对各平台都需要有对应的兼容，我们这边以linux为例，其他平台就大家自行去查阅了

bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {  assert(thread->osthread() == NULL, "caller responsible");  // Allocate the OSThread object  OSThread* osthread = new OSThread(NULL, NULL);  if (osthread == NULL) {    return false;  }  // set the correct thread state  osthread->set_thread_type(thr_type);  // Initial state is ALLOCATED but not INITIALIZED  osthread->set_state(ALLOCATED);  thread->set_osthread(osthread);  // init thread attributes  pthread_attr_t attr;  pthread_attr_init(&attr);  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);  // stack size  if (os::Linux::supports_variable_stack_size()) {    // calculate stack size if it"s not specified by caller    if (stack_size == 0) {      stack_size = os::Linux::default_stack_size(thr_type);      switch (thr_type) {      case os::java_thread:        // Java threads use ThreadStackSize which default value can be        // changed with the flag -Xss        assert (JavaThread::stack_size_at_create() > 0, "this should be set");        stack_size = JavaThread::stack_size_at_create();        break;      case os::compiler_thread:        if (CompilerThreadStackSize > 0) {          stack_size = (size_t)(CompilerThreadStackSize * K);          break;        } // else fall through:          // use VMThreadStackSize if CompilerThreadStackSize is not defined      case os::vm_thread:      case os::pgc_thread:      case os::cgc_thread:      case os::watcher_thread:        if (VMThreadStackSize > 0) stack_size = (size_t)(VMThreadStackSize * K);        break;      }    }    stack_size = MAX2(stack_size, os::Linux::min_stack_allowed);    pthread_attr_setstacksize(&attr, stack_size);  } else {    // let pthread_create() pick the default value.  }  // glibc guard page  pthread_attr_setguardsize(&attr, os::Linux::default_guard_size(thr_type));  ThreadState state;  {    // Serialize thread creation if we are running with fixed stack LinuxThreads    bool lock = os::Linux::is_LinuxThreads() && !os::Linux::is_floating_stack();    if (lock) {      os::Linux::createThread_lock()->lock_without_safepoint_check();    }    pthread_t tid;      //通过pthread_create方法创建内核级线程 ！    int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);    pthread_attr_destroy(&attr);    if (ret != 0) {      if (PrintMiscellaneous && (Verbose || WizardMode)) {        perror("pthread_create()");      }      // Need to clean up stuff we"ve allocated so far      thread->set_osthread(NULL);      delete osthread;      if (lock) os::Linux::createThread_lock()->unlock();      return false;    }    // Store pthread info into the OSThread    osthread->set_pthread_id(tid);    // Wait until child thread is either initialized or aborted    {      Monitor* sync_with_child = osthread->startThread_lock();      MutexLockerEx ml(sync_with_child, Mutex::_no_safepoint_check_flag);      while ((state = osthread->get_state()) == ALLOCATED) {        sync_with_child->wait(Mutex::_no_safepoint_check_flag);      }    }    if (lock) {      os::Linux::createThread_lock()->unlock();    }  }  // Aborted due to thread limit being reached  if (state == ZOMBIE) {      thread->set_osthread(NULL);      delete osthread;      return false;  }  // The thread is returned suspended (in state INITIALIZED),  // and is started higher up in the call chain  assert(state == INITIALIZED, "race condition");  return true;}

源码见：https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/69087d08d473/src/os/linux/vm/os_linux.cpp

主要通过pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread)，它是unix 创建线程的方法，linux也继承了。调用后在linux系统中会创建一个内核级的线程。也就是说这个时候操作系统中线程才真正地诞生

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但此时线程才诞生，那是怎么启动的？我们回到JVM_StartThread源码中，Thread::start(native_thread)很明显这行代码就表示启动native_thread = new JavaThread(&thread_entry, sz)创建的线程，我们来继续看看其源码

void Thread::start(Thread* thread) {  trace("start", thread);  // Start is different from resume in that its safety is guaranteed by context or  // being called from a Java method synchronized on the Thread object.  if (!DisableStartThread) {    if (thread->is_Java_thread()) {      // 设置线程状态      java_lang_Thread::set_thread_status(((JavaThread*)thread)->threadObj(),                                          java_lang_Thread::RUNNABLE);    }    os::start_thread(thread);  }}

源码：https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/69087d08d473/src/share/vm/runtime/thread.cpp

os::start_thread它封装了pd_start_thread(thread),执行该方法，操作系统会去启动指定的线程

void os::start_thread(Thread* thread) {  // guard suspend/resume  MutexLockerEx ml(thread->SR_lock(), Mutex::_no_safepoint_check_flag);  OSThread* osthread = thread->osthread();  osthread->set_state(RUNNABLE);  pd_start_thread(thread);}

当操作系统的线程启动完之后，我们再回到pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread)，会去java_start这个线程入口函数进行OS内核级线程的初始化，并开始启动JavaThread

// Thread start routine for all newly created threadsstatic void *java_start(Thread *thread) {  // Try to randomize the cache line index of hot stack frames.  // This helps when threads of the same stack traces evict each other"s  // cache lines. The threads can be either from the same JVM instance, or  // from different JVM instances. The benefit is especially true for  // processors with hyperthreading technology.  static int counter = 0;  int pid = os::current_process_id();  alloca(((pid ^ counter++) & 7) * 128);  ThreadLocalStorage::set_thread(thread);  OSThread* osthread = thread->osthread();  Monitor* sync = osthread->startThread_lock();  // non floating stack LinuxThreads needs extra check, see above  if (!_thread_safety_check(thread)) {    // notify parent thread    MutexLockerEx ml(sync, Mutex::_no_safepoint_check_flag);    osthread->set_state(ZOMBIE);    sync->notify_all();    return NULL;  }  // thread_id is kernel thread id (similar to Solaris LWP id)  osthread->set_thread_id(os::Linux::gettid());  if (UseNUMA) {    int lgrp_id = os::numa_get_group_id();    if (lgrp_id != -1) {      thread->set_lgrp_id(lgrp_id);    }  }  // initialize signal mask for this thread  os::Linux::hotspot_sigmask(thread);  // initialize floating point control register  os::Linux::init_thread_fpu_state();  // handshaking with parent thread  {    MutexLockerEx ml(sync, Mutex::_no_safepoint_check_flag);    // notify parent thread    osthread->set_state(INITIALIZED);    sync->notify_all();    // 等待,直到操作系统级线程全部启动    while (osthread->get_state() == INITIALIZED) {      sync->wait(Mutex::_no_safepoint_check_flag);    }  }  // 开始运行JavaThread::run  thread->run();  return 0;}

源码：https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/69087d08d473/src/os/linux/vm/os_linux.cpp

thread->run()其实就是JavaThread::run()也表明方法开始回调，从OS层方法回到JVM层方法，我们再来看下其实现：

// The first routine called by a new Java threadvoid JavaThread::run() {  // initialize thread-local alloc buffer related fields  this->initialize_tlab();  // used to test validitity of stack trace backs  this->record_base_of_stack_pointer();  // Record real stack base and size.  this->record_stack_base_and_size();  // Initialize thread local storage; set before calling MutexLocker  this->initialize_thread_local_storage();  this->create_stack_guard_pages();  this->cache_global_variables();  // Thread is now sufficient initialized to be handled by the safepoint code as being  // in the VM. Change thread state from _thread_new to _thread_in_vm  ThreadStateTransition::transition_and_fence(this, _thread_new, _thread_in_vm);  assert(JavaThread::current() == this, "sanity check");  assert(!Thread::current()->owns_locks(), "sanity check");  DTRACE_THREAD_PROBE(start, this);  // This operation might block. We call that after all safepoint checks for a new thread has  // been completed.  this->set_active_handles(JNIHandleBlock::allocate_block());  if (JvmtiExport::should_post_thread_life()) {    JvmtiExport::post_thread_start(this);  }  JFR_ONLY(Jfr::on_thread_start(this);)  // We call another function to do the rest so we are sure that the stack addresses used  // from there will be lower than the stack base just computed  thread_main_inner();//!!!注意此处方法  // Note, thread is no longer valid at this point!}void JavaThread::thread_main_inner() {  assert(JavaThread::current() == this, "sanity check");  assert(this->threadObj() != NULL, "just checking");  // Execute thread entry point unless this thread has a pending exception  // or has been stopped before starting.  // Note: Due to JVM_StopThread we can have pending exceptions already!  if (!this->has_pending_exception() &&      !java_lang_Thread::is_stillborn(this->threadObj())) {    {      ResourceMark rm(this);      this->set_native_thread_name(this->get_thread_name());    }    HandleMark hm(this);    this->entry_point()(this, this);//JavaThread对象中传入的entry_point为Thread对象的Thread::run方法  }  DTRACE_THREAD_PROBE(stop, this);  this->exit(false);  delete this;}

源码：https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/69087d08d473/src/share/vm/runtime/thread.cpp

由于JavaThread定义可知JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz)中参数entry_point是外部传入，那我们想想JavaThread是什么时候实例化的？

没错，就是我们一开始的JVM_StartThread中native_thread = new JavaThread(&thread_entry, sz);也就是说this->entry_point()(this, this)实际上是回调的thread_entry方法

thread_entry源码：

static void thread_entry(JavaThread* thread, TRAPS) {  HandleMark hm(THREAD);  Handle obj(THREAD, thread->threadObj());  JavaValue result(T_VOID);  JavaCalls::call_virtual(&result,                          obj,                          KlassHandle(THREAD, SystemDictionary::Thread_klass()),                          vmSymbols::run_method_name(),                          vmSymbols::void_method_signature(),                          THREAD);}

源码：https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/69087d08d473/src/share/vm/prims/jvm.cpp通过JavaCalls::call_virtual方法，又从JVM层 回到了Java语言层 ，即MyThread thread = new MyThread(); thread.start();

一切又回到了起点，这就是Javathread.start()内部完整的一个流程，HotSpot虚拟机实现的Java线程其实是对Linux内核级线程的直接映射，将Java涉及到的所有线程调度、内存分配都交由操作系统进行管理。

线程终止状态(TERMINATED)，表示该线程已经运行完毕。

当一个线程执行完毕，或者主线程的main()方法完成时，我们就认为它终止了。终止的线程无法在被使用，如果调用start()方法，会抛出java.lang.IllegalThreadStateException异常，这一点我们可以从start源码中很容易地得到

public synchronized void start() {    if (threadStatus != 0)        throw new IllegalThreadStateException();    ...}

线程阻塞状态(BLOCKED)，需要等待锁释放或者说获取锁失败时，线程阻塞

public class BlockedThread implements Runnable {    @Override    public void run() {        synchronized (BlockedThread.class){            while (true){                            }        }    }}

从Thread源码的注释中，我们可以知道等待锁释放或者说获取锁失败，主要有下面3中情况：

其中第三种情况，大家可以先思考一下，我们留在下文线程等待状态再详细展开

线程等待状态(WAITING)，表示该线程需要等待其他线程做出一些特定动作（通知或中断）。

我们紧接着上一小节，调用wait 后, 重新进入synchronized 方法/块时，我们来看看期间发生了什么？

当线程1调用对象A的wait方法后，会释放当前的锁，然后让出CPU时间片，线程会进入该对象的等待队列中，线程状态变为 等待状态WAITING。当另一个线程2调用了对象A的notify()/notifyAll()方法

notify（）方法只会唤醒沉睡的线程，不会立即释放之前占有的对象A的锁，必须执行完notify()方法所在的synchronized代码块后才释放。所以在编程中，尽量在使用了notify/notifyAll()后立即退出临界区

线程1收到通知后退出等待队列，并进入线程运行状态RUNNABLE，等待 CPU 时间片分配, 进而执行后续操作，接着线程1重新进入 synchronized 方法/块时，竞争不到锁，线程状态变为线程阻塞状态BLOCKED。如果竞争到锁，就直接接着运行。线程等待状态 切换到线程阻塞状态，无法直接切换，需要经过线程运行状态。

我们再来看一个例子，巩固巩固：

public class WaitNotifyTest {    public static void main(String[] args) {        Object A = new Object();        new Thread(new Runnable() {            @Override            public void run() {                System.out.println("线程1等待获取 对象A的锁...");                synchronized (A) {                    try {                        System.out.println("线程1获取了 对象A的锁");                        Thread.sleep(3000);                        System.out.println("线程1开始运行wait()方法进行等待，进入到等待队列......");                        A.wait();                        System.out.println("线程1等待结束");                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                }            }        }).start();        new Thread(new Runnable() {            @Override            public void run() {                System.out.println("线程2等待获取 对象A的锁...");                synchronized (A) {                    System.out.println("线程2获取了 对象A的锁");                    try {                        Thread.sleep(3000);                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                    System.out.println("线程2将要运行notify()方法进行唤醒线程1");                    A.notify();                }            }        }).start();    }}

结果：

线程1等待获取 对象A的锁...线程1获取了 对象A的锁线程2等待获取 对象A的锁...线程1开始运行wait()方法进行等待，进入到等待队列......线程2获取了 对象A的锁线程2将要运行notify()方法进行唤醒线程1线程1等待结束

需要注意的是，wait/notify/notifyAll 只能在synchronized修饰的方法、块中使用， notify 是只随机唤醒一个线程，而 notifyAll 是唤醒所有等待队列中的线程

Thread类中的join方法的主要作用能让线程之间的并行执行变为串行执行，当前线程等该加入该线程后面，等待该线程终止

public static void main(String[] args) {  Thread thread = new Thread();  thread.start();  thread.join();  ...}

上面一个例子表示，程序在main主线程中调用thread线程的join方法,意味着main线程放弃CPU时间片(主线程会变成 WAITING 状态)，并返回thread线程，继续执行直到线程thread执行完毕，换句话说在主线程执行过程中，插入thread线程，还得等thread线程执行完后，才轮到主线程继续执行

如果查看JDKthread.join()底层实现，会发现其实内部封装了wait(),notifyAll()

LockSupport.park() 挂起当前线程；LockSupport.unpark(暂停线程对象) 恢复某个线程

package com.zj.ideaprojects.demo.test3;import java.util.concurrent.Executors;import java.util.concurrent.locks.LockSupport;public class ThreadLockSupportTest {    public static void main(String[] args) throws InterruptedException {        Thread thread = new Thread(() -> {            System.out.println("start.....");            try {                Thread.sleep(1000);            } catch (InterruptedException e) {                e.printStackTrace();            }            System.out.println("park....");            LockSupport.park();            System.out.println("resume.....");        });        thread.start();        Thread.sleep(3000);        System.out.println("unpark....");        LockSupport.unpark(thread);    }}

结果：

start.....park....unpark....resume.....

当程序调用LockSupport.park()，会让当前线程A的线程状态会从 RUNNABLE 变成 WAITING，然后main主线程调用LockSupport.unpark(thread)，让指定的线程即线程A,从 WAITING 回到 RUNNABLE 。我们可以发现park/unpark和wait/notify/notifyAll很像，但是他们有以下的区别：

超时等待状态(TIMED_WAITING)，也叫限期等待，可以在指定的时间后自行返回而不是像 WAITING 那样一直等待。

这部分比较简单，它和线程等待状态(WAITING)状态 非常相似，区别就是方法的参数舒服传入限制时间，在 Timed Waiting状态时会等待超时，之后由系统唤醒，或者也可以提前被通知唤醒如 notify

相关方法主要有：

1. Object.wait(long)2. Thread.join(long) 3. LockSupport.parkNanos(long)4. LockSupport.parkUntil(long)5. Thread.sleep(long)

需要注意的是Thread.sleep(long),当线程执行sleep方法时，不会释放当前的锁(如果当前线程进入了同步锁)，也不会让出CPU。sleep(long)可以用指定时间使它自动唤醒过来，如果时间不到只能调用interrupt方法强行打断。

参考资料：

https://hg.openjdk.java.net/jdk8u

《并发编程的艺术》

https://www.jianshu.com/p/216a41352fd8

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原文镜像：原来还能这样看Java线程的状态及转换

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