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Java这20年改了好多。虽然很保守，但一直有新特性加进来。请甄别下面这些特性是哪个版本加进Java语言的。其中有些特性实际上属于JVM或者Java API，这里都归在一起了。 0. synchronized关键字。每个Java对象里都有一个隐藏的reentrant锁。synchronized语句可以获得这个锁，进行安全的互斥操作，还可以进行条件提醒。 package cn.byr.nuanyangyang.java20.synch; class Foo { int num = 0; final int limit = 10; public synchronized void foo() { num = num + 1; if (num >= limit) { notifyAll(); } } public void bar() { int mynum; synchronized (this) { mynum = num; } System.out.println(mynum); } public void baz() throws InterruptedException { int mynum; synchronized (this) { while ((mynum = num) < limit) { wait(); } } System.out.println("mynum = " + mynum + ", which is >= limit."); } } public class SynchTest { public static void main(String[] args) throws Exception { final Foo f = new Foo(); f.foo(); f.bar(); Thread waiterThread = new Thread(() -> { try { System.out.println("Waiting for mynum to exceed limit..."); f.baz(); // May see mynum > 10. That's normal because // waiterThread may actually start later. System.out.println("Finished waiting."); } catch (Exception e) { System.out.println("Oops. I am interrupted! I should stop."); } }); waiterThread.start(); for (int i = 1; i < 15; i++) { f.foo(); f.bar(); } waiterThread.join(); } } 1. reflect（反射）：Java可以在运行时获得一个对象的类型信息，包括类的名称、父类、成员变量、方法，并可以动态地创建实例，也可以动态地调用方法。这使得Java在某些方面可以像Python一样的动态语言一样进行运行时动态操作。 package cn.byr.nuanyangyang.java20.reflect; import java.lang.reflect.Method; class Person { private String name; private int age; private double salary; public Person(String name) { this.name = name; } public String getName() { return name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public double getSalary() { return salary; } public void setSalary(double salary) { this.salary = salary; } } public class ReflectTest { public static void main(String[] args) throws Exception { Person p = new Person("Alice"); p.setAge(30); p.setSalary(5000.0); Class<? extends Person> personCls = p.getClass(); System.out.println(personCls.getName()); for (Method m : personCls.getMethods()) { String name = m.getName(); if (name.startsWith("get") && m.getParameterCount() == 0) { Object value = m.invoke(p); System.out.format("%s: %s\n", name, value); } } } } 2. Swing图形界面：Java支持图形界面编程，适合创建跨平台的、多线程的图形界面程序。 package cn.byr.nuanyangyang.java20.swing; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.SwingUtilities; public class SwingTest { public static void main(String[] args) { JFrame f = new JFrame(); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); f.setTitle("Hello world"); final JLabel l = new JLabel(); l.setText("Counting here..."); f.add(l); f.pack(); f.setVisible(true); Thread countingThread = new Thread(() -> { try { for (int i = 0;; i++) { final int j = i; SwingUtilities.invokeLater(() -> { l.setText(String.format("i = %d", j)); }); Thread.sleep(1000); } } catch (InterruptedException e) { // should stop } }); countingThread.setDaemon(true); countingThread.start(); } } 3. Proxy类：Java可以在运行时创建Proxy类，实现给定的接口。这样，可以通过反射的方式动态实现一个接口。比如，截获调用的方法，计入日志，或者转换成网络调用。 package cn.byr.nuanyangyang.java20.proxy; import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; interface Calculator { double add(double a, double b); double sub(double a, double b); double sqrt(double a); } public class ProxyTest { public static void main(String[] args) throws Exception { Calculator fakeCalc = (Calculator) Proxy.newProxyInstance(ClassLoader.getSystemClassLoader(), new Class<?>[] { Calculator.class }, new InvocationHandler() { @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.format("You called method: %s\n", method.getName()); for (Object arg : args) { System.out.format(" Arg: %s, type: %s\n", arg, arg.getClass()); } return 42.0; } }); fakeCalc.add(1.0, 2.0); fakeCalc.sub(3.0, 4.0); fakeCalc.sqrt(2.0); } }

[Java 20年]Quiz: 下列特性是哪个Java版本引入的？(1)