Java 关键字【synchronized】

修饰代码块

使用synchronized关键字来修饰一个代码块，语法为synchronized (对象表达式) {代码块}

// 源码 java.lang.ClassLoader.class    
protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        // 为了保证类加载过程的原子性和正确性，使用了synchronized
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);

                    // this is the defining class loader; record the stats
                    PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

修饰方法：

当 synchronized 修饰一个实例方法时，它会锁定当前对象的实例

 // 源码 Thread类中的start()方法 
/** 
使用 synchronized 修饰 Thread 的 start 方法是为了保证线程状态转换的一致性、线程组操作的安全性以及避免并发执行 start0 方法，确保线程启动的过程是线程安全的，避免多个线程并发修改线程相关状态和调用 start0 方法导致的各种错误和异常。这种同步机制保证了 Thread 类在多线程环境下的正常工作和正确使用。
**/
 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 */
            }
        }
    }