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synchronizedsynchronized (getClassLoadingLock(name)) {// First, check if the class has already been loadedClass<?> 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 statsPerfCounter.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 thenit will be passed up the call stack */}}}