线程之Thread

Posted 2020-12-11 zby9527

如何创建一个线程并启动

        // 1.创建一个Thread类的实例对象
        Thread thread = new Thread();
        // 2.调用Thread实例对象的start方法
        thread.start();

• 线程启动后会在一个新的线程中运行线程对象的run方法

    @Override
    public void run() {
        if (target != null) {
            target.run();
        }
    }

• 因此，要在新的线程中运行用户代码，有两种方式
  • 继承Thread类并重写run方法
  • 构造Thread对象时，传入Runnable对象，替换原有的target对象

Thread详解

构造方法

    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);
    }

• 都是通过init方法构造线程对象
• group：指定所属线程组
• target：替换target对象
• name：指定线程名称

init方法

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();
        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();
            }
        }

        /* 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();
        this.priority = parent.getPriority();
        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();
    }

• 创建的线程对象的parent为创建当前线程对象的线程
• 所属线程组默人为parent所属线程组
• 线程是否时守护线程默认继承自parent
• 线程优先级默认继承自parent
• 线程类加载器默认继承自parent
• 线程inheritableThreadLocals继承自parent

线程状态

public enum State {
        /**
         * Thread state for a thread which has not yet started.
         */
        NEW,

        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
         */
        RUNNABLE,

        /**
         * Thread state for a thread blocked waiting for a monitor lock.
         * A thread in the blocked state is waiting for a monitor lock
         * to enter a synchronized block/method or
         * reenter a synchronized block/method after calling
         * {@link Object#wait() Object.wait}.
         */
        BLOCKED,

        /**
         * Thread state for a waiting thread.
         * A thread is in the waiting state due to calling one of the
         * following methods:
         * <ul>
         *   <li>{@link Object#wait() Object.wait} with no timeout</li>
         *   <li>{@link #join() Thread.join} with no timeout</li>
         *   <li>{@link LockSupport#park() LockSupport.park}</li>
         * </ul>
         *
         * <p>A thread in the waiting state is waiting for another thread to
         * perform a particular action.
         *
         * For example, a thread that has called <tt>Object.wait()</tt>
         * on an object is waiting for another thread to call
         * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
         * that object. A thread that has called <tt>Thread.join()</tt>
         * is waiting for a specified thread to terminate.
         */
        WAITING,

        /**
         * Thread state for a waiting thread with a specified waiting time.
         * A thread is in the timed waiting state due to calling one of
         * the following methods with a specified positive waiting time:
         * <ul>
         *   <li>{@link #sleep Thread.sleep}</li>
         *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
         *   <li>{@link #join(long) Thread.join} with timeout</li>
         *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
         *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
         * </ul>
         */
        TIMED_WAITING,

        /**
         * Thread state for a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }

异常处理

• Thread

    private void dispatchUncaughtException(Throwable e) {
        getUncaughtExceptionHandler().uncaughtException(this, e);
    }
    public UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return uncaughtExceptionHandler != null ?
            uncaughtExceptionHandler : group;
    }

• ThreadGroup

    public void uncaughtException(Thread t, Throwable e) {
        if (parent != null) {
            parent.uncaughtException(t, e);
        } else {
            Thread.UncaughtExceptionHandler ueh =
                Thread.getDefaultUncaughtExceptionHandler();
            if (ueh != null) {
                ueh.uncaughtException(t, e);
            } else if (!(e instanceof ThreadDeath)) {
                System.err.print("Exception in thread ""
                                 + t.getName() + "" ");
                e.printStackTrace(System.err);
            }
        }
    }

• Thread异常由异常处理器处理，默认为null则交给ThreadGroup处理
• ThreadGroup实现了Thread.UncaughtExceptionHandler
• Thread没有默认的异常处理器
• 自己处理异常只需要实现UncaughtExceptionHandler并调用setUncaughtExceptionHandler

线程优先级

    /**
     * The minimum priority that a thread can have.
     */
    public final static int MIN_PRIORITY = 1;

   /**
     * The default priority that is assigned to a thread.
     */
    public final static int NORM_PRIORITY = 5;

    /**
     * The maximum priority that a thread can have.
     */
    public final static int MAX_PRIORITY = 10;

• 并不是一定严格按照优先级调度线程，只能说优先级高调度机会高

线程组

• 批量管理一组线程
• 实现UncaughtExceptionHandler，提供默认的异常处理

线程中断

    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();
    }

    public static boolean interrupted() {
        return currentThread().isInterrupted(true);
    }

    public boolean isInterrupted() {
        return isInterrupted(false);
    }

    private native boolean isInterrupted(boolean ClearInterrupted);

• interrupt设置中断标志
• interrupted返回是否中断并清除标识位
• isInterrupted返回是否中断不清除标识位

Deprecated方法

• stop、destroy、suspend、resume、countStackFrames

• 会引发死锁，避免使用

• 线程挂起恢复使用sleep、join或者Object的wait、notify等实现

线程join()方法

• see https://www.cnblogs.com/zby9527/p/7526606.html

ThreadLocal

• Thread

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

• inheritableThreadLocals在构造方法里介绍了

• ThreadLocal

    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

• 创建一个Map，赋值给Thread对象的threadLocals变量

    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
    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();
    }

• 以当前ThreadLocal对象为key，对map进行操作
• 看懂上面代码的this
• ThreadLocal中有一个神奇的数字0x61c88647，Why 0x61c88647

ThreadFactory

• java.util.concurrent.ThreadFactory

package java.util.concurrent;

public interface ThreadFactory {

    Thread newThread(Runnable r);
}

• java.util.concurrent.Executors.DefaultThreadFactory

    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;

        DefaultThreadFactory() {
            SecurityManager s = System.getSecurityManager();
            group = (s != null) ? s.getThreadGroup() :
                                  Thread.currentThread().getThreadGroup();
            namePrefix = "pool-" +
                          poolNumber.getAndIncrement() +
                         "-thread-";
        }

        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;
        }
    }

• 抄一个线程工程，把namePrefix的pool改成自己业务域名称