java多线程总结 - 基础篇

男娘i 2021-06-10 20:39 402阅读 0赞

#### 进程与线程区别 ####

进程是资源分配的最小单位，进程中会有多个线程。引入线程的目的是因为计算机cpu上下文切换会很频繁，而进程的上下文切换相对耗时耗资源。  
而线程是在进程中的，线程是cpu调度的最小单位，进程中的多个线程共享一块内存。线程的上下文切换成本更低。

#### 线程的状态 ####

New、Runnable（包含运行态和就绪态）、Waiting、TimeWaiting、Blocked、Terminated

#### 线程的start与run方法的区别 ####

start：新创建一个线程并执行run方法  
run：在当前线程执行run方法  
start方法源码：

/** * Causes this thread to begin execution; the Java Virtual Machine * calls the <code>run</code> method of this thread. * * The result is that two threads are running concurrently: the * current thread (which returns from the call to the * <code>start</code> method) and the other thread (which executes its * <code>run</code> method). * * It is never legal to start a thread more than once. * In particular, a thread may not be restarted once it has completed * execution. * @exception IllegalThreadStateException if the thread was already * started. * @see #run() * @see #stop() */
 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 { 
 	// 执行本地方法，新创建一个线程，并执行run方法
 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();

JVM对应的源码：http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/63dd5b7d50eb/src/share/native/java/lang/Thread.c  
调用了JVM\_StartThread方法：

{ "start0",           "()V",        (void *)&JVM_StartThread},

http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/176a7e5cc060/src/share/vm/prims/jvm.cpp  
JVM\_StartThread方法，new JavaThread(&thread\_entry, sz)：

JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
      JVMWrapper("JVM_StartThread");
      JavaThread *native_thread = NULL;
    
      // We cannot hold the Threads_lock when we throw an exception,
      // due to rank ordering issues. Example: we might need to grab the
      // Heap_lock while we construct the exception.
      bool throw_illegal_thread_state = false;
    
      // We must release the Threads_lock before we can post a jvmti event
      // in Thread::start.
      { 
        // Ensure that the C++ Thread and OSThread structures aren't freed before
        // we operate.
        MutexLocker mu(Threads_lock);
    
        // Since JDK 5 the java.lang.Thread threadStatus is used to prevent
        // re-starting an already started thread, so we should usually find
        // that the JavaThread is null. However for a JNI attached thread
        // there is a small window between the Thread object being created
        // (with its JavaThread set) and the update to its threadStatus, so we
        // have to check for this
        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));
          // Allocate the C++ Thread structure and create the native thread. The
          // stack size retrieved from java is signed, but the constructor takes
          // size_t (an unsigned type), so avoid passing negative values which would
          // result in really large stacks.
          size_t sz = size > 0 ? (size_t) size : 0;
          // !!!! 调用了JVM_StartThread的new JavaThread
          native_thread = new JavaThread(&thread_entry, sz);
    
          // At this point it may be possible that no osthread was created for the
          // JavaThread due to lack of memory. Check for this situation and throw
          // an exception if necessary. Eventually we may want to change this so
          // that we only grab the lock if the thread was created successfully -
          // then we can also do this check and throw the exception in the
          // JavaThread constructor.
          if (native_thread->osthread() != NULL) { 
            // Note: the current thread is not being used within "prepare".
            native_thread->prepare(jthread);
          }
        }
      }

run方法源码：单纯在当前线程中执行run方法

/** * If this thread was constructed using a separate * <code>Runnable</code> run object, then that * <code>Runnable</code> object's <code>run</code> method is called; * otherwise, this method does nothing and returns. * * Subclasses of <code>Thread</code> should override this method. * * @see #start() * @see #stop() * @see #Thread(ThreadGroup, Runnable, String) */
 @Override
 public void run() { 
 if (target != null) { 
 target.run();
 }
 }

#### 创建线程有哪些方式？ ####

1、继承Thread类（Thread类也实现了Runnable接口），重写run方法  
2、实现Runnable接口，重写run方法，Thread threadA = new Thread(Runnable r);  
两者本质上都是创建Thread对象，通过start方法，创建一个新线程并执行thread对象的run方法。

#### 如何处理线程的返回值？ ####

可以通过JUC包下的FutureTask，我们知道Thread的run方法是没有返回值的，而FutureTask实现了Callable接口，同时也实现了Runnable接口。Callable接口是有返回值的。

public interface Callable<V> { 
 /** * Computes a result, or throws an exception if unable to do so. * * @return computed result * @throws Exception if unable to compute a result */
 V call() throws Exception;
 }

FutureTask通过实现Callable接口，将Runnable的run方法关联起来，并将线程的返回值通过set()方法保存在一个outcome的对象中。通过get()方法可以获取到这个返回值。  
下面是FutureTask类的run方法部分代码：

public void run() { 
 try { 
 	// 取得callable的实现
 Callable<V> c = callable;
 if (c != null && state == NEW) { 
 V result;
 boolean ran;
 try { 
 	// 执行call方法，将返回值存在临时变量result
 result = c.call();
 ran = true;
 } catch (Throwable ex) { 
 result = null;
 ran = false;
 setException(ex);
 }
 if (ran)
 	// 将result保存到outcome字段
 set(result);
 }
 }
 }
 	//保存到outcome字段
 protected void set(V v) { 
 outcome = v;
 }

#### sleep与wait的区别 ####

sleep是Thread类的方法，wait是Object类的方法。  
sleep方法可以随时被调用，而wait方法只能在synchronized中使用（你只有持有锁，才能释放锁）  
sleep是定时等待，有时间设定，在此期间，线程只释放cpu，不释放同步资源锁。  
wait是无限期等待，释放cpu，同时释放所持有的的锁，只有等notify或notifyAll时被唤醒。

#### notify与notifyAll的区别 ####

锁池：当一个线程获取到对象锁之后，其他线程去竞争这把锁之后，会进入阻塞状态，并进入锁池。等待被唤醒。  
notify：随机唤醒一个阻塞线程去竞争锁  
notifyAll：唤醒所有阻塞线程，去竞争锁

#### yield函数 ####

/** * A hint to the scheduler that the current thread is willing to yield * its current use of a processor. The scheduler is free to ignore this * hint. * * Yield is a heuristic attempt to improve relative progression * between threads that would otherwise over-utilise a CPU. Its use * should be combined with detailed profiling and benchmarking to * ensure that it actually has the desired effect. * * It is rarely appropriate to use this method. It may be useful * for debugging or testing purposes, where it may help to reproduce * bugs due to race conditions. It may also be useful when designing * concurrency control constructs such as the ones in the * {@link java.util.concurrent.locks} package. */
 public static native void yield();

yield函数主要用于告诉cpu，我愿意将cpu资源让给其他线程，但这也可能被调度器忽略。主要用于多线程锁相关的问题。

#### interrupt函数 ####

interrupt是中断的意思，例如我们鼠标和键盘向计算机输入信息，就是通过中断实现的。这里的interrupt是向线程发起一个中断信号，这样就可以将线程进行标记。如果接收到终端信号的线程处于阻塞状态，则抛出InterruptException异常，后续可以对该线程的中断进行相应的处理。  
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