java线程基础巩固---构造Thread对象你也许不知道的几件事

喜欢ヅ旅行 2021-09-29 12:48 275阅读 0赞

关于Thread的构造在JDK文档中如下：

![324374-20171108210332419-1822062164.png][]

之后会把上面所有的构造都会学习到，这次主要是去研究一下图上标红的默认构造，当然大家肯定对于它都有些不屑，这有啥可学的，不new一个然后start线程不就启动了，属于线程最最基础的东东，然后它的背后会并非我们都知道，所以下面开始学习。

**Thread常规知识：**

先来看一下默认构造的源码：

![324374-20171108210858231-1537072490.png][]

先了解一下默认线程名的起名规则，看下nextThreadNum()：

![324374-20171108211043122-2054419495.png][]

原来是有一个静态计数的变量，那也就是说默认new出来的线程是以"Thread-0、Thead-1"这样的规则来命名的，用代码来验证下：

![324374-20171108211348044-355888349.png][]

编译运行:

![324374-20171108211407919-337510765.png][]

果真如预期，但是目前线程中并未做任何事情，而我们知道可以给线程构造中传一个Runnable接口：

![324374-20171108212533481-122120897.png][]

那在默认构造Thread的时候有没有一个空的Runnable去执行呢?看源码【贴出关键代码】：

public Thread() {
            init(null, null, "Thread-" + nextThreadNum(), 0);
        }
    
        private void init(ThreadGroup g, Runnable target, String name,
                          long stackSize) {
            init(g, target, name, stackSize, null);
        }

从代码中可以看到并未自动创建一个Runnable，而是直接传的null，那最终在init()中是如何搞的呢？

private void init(ThreadGroup g, Runnable target, String name,
                          long stackSize, AccessControlContext acc) {
            if (name == null) {
                throw new NullPointerException("name cannot be null");
            }
    
            this.name = name.toCharArray();
    
            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 (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();
        }

只看核心代码，可以看到将Thread的成员变量target赋值为null了，

![324374-20171108213130153-27243936.png][]

那此时我们执行创建的线程会调用start()方法，那接着看下它：

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 {
                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();

最终转到C层去执行线程的启动了，而底层的代码暂且不深纠了，这里要知道C层会去调用Thread的run()方法，所以转到run方法去看一眼：

![324374-20171108213445341-619198014.png][]

但是！！！可以重写它的run方法来执行我们的动作，如下：

![324374-20171108213644106-853582555.png][]

编译运行：

![324374-20171108213706716-1054236042.png][]

那接着修改代码：

![324374-20171108214533513-2025913762.png][]

![324374-20171108214633216-257972173.png][]

![324374-20171108214909559-685485611.png][]

接着在创建runnable的时候也手动给它取个名字，如下：

![324374-20171108215247872-1890654939.png][]

![324374-20171108215306700-1861598292.png][]

这样关于Thread的构造就学习了四种了，当然也是最经常使用的，比较简单：

![324374-20171108215401309-1938573742.png][]

**ThreadGroup的概念及守护线程初探：**

关于这个内容可能就不是太清楚啦，可以看一下剩下线程构造函数中：

![324374-20171108220840559-1859638527.png][]

全是跟ThreadGroup相关，貌似实际工作中完全木有接触过它，所以说首先得了解它，从哪了解呢，直接从源码：

public Thread() {
       init(null, null, "Thread-" + nextThreadNum(), 0);
    }
    
    private void init(ThreadGroup g, Runnable target, String name,
                          long stackSize, AccessControlContext acc) {
            if (name == null) {
                throw new NullPointerException("name cannot be null");
            }
    
            this.name = name.toCharArray();
    
            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 (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();
        }

从源码中可以看到，默认无参的构造函数并未指定ThreadGroup，当在执行init()方法时发现没有ThreadGroup，则会获得父线程的ThreadGroup，而父线程可以通过currentThread()方法获取，而我们在之前【[http://www.cnblogs.com/webor2006/p/7682063.html][http_www.cnblogs.com_webor2006_p_7682063.html]】有介绍过，对于创建一个线程实际是有两个线程，其中有一个就是main线程，也就是用来启动我们自己创建的线程的，那是不是我们线程的parent就是main呢？下面来做实验论证：

![324374-20171108222019825-1436554649.png][]

![324374-20171108222052419-446816716.png][]

那如果此时新建一个线程，然后再看一下它的ThreadGroup，如下：

![324374-20171108222336419-294052287.png][]

![324374-20171108222404059-1749633832.png][]

那说明创建线程时未指定ThreadGroup，那会以父线程的ThreadGroup作为该线程的ThreadGroup，刚好跟源码逻辑吻和，那是不是说子线程将和父线程在同一个ThreadGroup中？也就是说目前ThreadGroup中应该包含了2个线程，那我们打印看一下：

![324374-20171112152937747-530199696.png][]

编译运行：

![324374-20171112153022809-1701487151.png][]

这是在MAC上的表现，但是！！！如果在windows平台其输出并非如我们所预期的，下面在windows下运行看一下结果：

![324374-20171112153309263-452891456.png][]

![324374-20171112153530778-469206334.png][]

**总结：**

*  创建线程对象Thread，默认有一个线程名，以Thread-开头，从0开始计数：Thread-0、Thread-1、Thread-2等。
 *  如果在构造Thread()的时候没有传递Runnable或者没有复写Thread的run方法，该Thread将不会调用任何东西；如果传递了Runnable接口的实例，或者复写的Thread的run方法，则会执行该方法的逻辑单元（逻辑代码）。
 *  如果构造线程对象时未传入ThreadGroup,Thread会默认获取父线程的TreadGroup作为该线程的ThreadGroup，此时子线程将和父线程在同一个ThreadGroup中。

转载于:https://www.cnblogs.com/webor2006/p/7760422.html

[324374-20171108210332419-1822062164.png]: /images/20210725/339e4fd3e372438b85b5704aca7a9d30.png
[324374-20171108210858231-1537072490.png]: /images/20210725/33293c15d7314f8fa3bd64a1713f5123.png
[324374-20171108211043122-2054419495.png]: /images/20210725/465d15ed9a6143ab8a5bfb91c5b70182.png
[324374-20171108211348044-355888349.png]: /images/20210725/b806bc2bf63640489485ce3021aae91a.png
[324374-20171108211407919-337510765.png]: /images/20210725/b574de6638244689925491d7cb2fb28e.png
[324374-20171108212533481-122120897.png]: /images/20210725/f242acc6e69546a394e6cb675d6935cb.png
[324374-20171108213130153-27243936.png]: /images/20210725/cc3e2e2db8e849c58c5d94a2e05bd1fb.png
[324374-20171108213445341-619198014.png]: /images/20210725/52956067dd094dd0a6dbc61c8ee1c9ce.png
[324374-20171108213644106-853582555.png]: /images/20210725/20ad02bb33f9436990ae64680efa68b9.png
[324374-20171108213706716-1054236042.png]: /images/20210725/8361d73a75f64b4f8a219b43470cbd85.png
[324374-20171108214533513-2025913762.png]: /images/20210725/0085387e1b8a4e97b206ccdfb1c1958e.png
[324374-20171108214633216-257972173.png]: /images/20210725/cecbab43776a417d88c21b302e471dd4.png
[324374-20171108214909559-685485611.png]: /images/20210725/f9d260cd5f1e470581edfeaf73c8db0a.png
[324374-20171108215247872-1890654939.png]: /images/20210725/0b2d938f2e0147c6b828e38c4006f5f6.png
[324374-20171108215306700-1861598292.png]: /images/20210725/dc3db0ba512f468cbdc832a6b143e7e5.png
[324374-20171108215401309-1938573742.png]: /images/20210725/b76b5649cadc40ecb6ff80b7ebdeaa5b.png
[324374-20171108220840559-1859638527.png]: /images/20210725/685817a063f640f6a07bc06c42149670.png
[http_www.cnblogs.com_webor2006_p_7682063.html]: http://www.cnblogs.com/webor2006/p/7682063.html
[324374-20171108222019825-1436554649.png]: /images/20210725/df8f03e5ef954990b1da8fddac6874fb.png
[324374-20171108222052419-446816716.png]: /images/20210725/01ade4dd96774056ba860d228db37a86.png
[324374-20171108222336419-294052287.png]: /images/20210725/d5c91e75b79c422480f8e470c6b66271.png
[324374-20171108222404059-1749633832.png]: /images/20210725/16c0412e1d194dd9b4faf3ecee039284.png
[324374-20171112152937747-530199696.png]: /images/20210725/8b527a86c836487eb586bd9c1ae01ce2.png
[324374-20171112153022809-1701487151.png]: /images/20210725/8e663c6955744c7992b92358a28e9e74.png
[324374-20171112153309263-452891456.png]: /images/20210725/3abc11f5009547db99baf821a0cc7495.png
[324374-20171112153530778-469206334.png]: /images/20210725/5c23cfd4cee941998c1c3c07361d1a8d.png