运行期间，获取类的信息，进行一些操作。
- 运行时构造类的对象。
- 运行时获取类的成员变量和方法。
- 运行时调用对象的方法（属性）。
Class 类封装了类的所有信息。
//1.类名.class -> Person.class
//2.对象.getClass() -> person.getClass()
//3.Class.forName(类全名) -> Class.forName("com.yoyiyi.test.Person")Person.java
public class Person {
String name;
private int age;
public Person(String name, int age) {
super();
this.name = name;
this.age = age;
System.out.println("有参数构造器");
}
public Person() {
super();
System.out.println("无参数构造器");
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
private void privateMethod() {
System.out.println("私有方法");
}
}
public class TestReflec {
public static void main(String[] args) throws Exception {
Class<Person> clazz = (Class<Person>) Class.forName("demo02.Person");
/***1.构造器**/
//获取所有的构造器
Constructor<Person>[] constructors =
(Constructor<Person>[]) clazz.getConstructors();
for (Constructor<Person> c : constructors) {
System.out.println(c);
}
//获取某一个构造器
Constructor<Person> constructor = clazz.getConstructor(String.class, int.class);
//创建对象
constructor.newInstance("Jack", 89);
/**2.方法**/
//获取类所有方法，包括父类，私有方法不能获取到
Method[] methods = clazz.getMethods();
for (Method method : methods) {
System.out.println(method.getName());
}
//获取当前类所有方法，包括私有方法
Method[] methods1 = clazz.getDeclaredMethods();
for (Method method : methods1) {
System.out.println(method.getName());
}
//获取指定方法
Method setName = clazz.getDeclaredMethod("setName", String.class);
Method setAge = clazz.getDeclaredMethod("setAge", int.class);
System.out.println(setName.getName());
System.out.println(setAge.getName());
//调用某一个方法
Person person = clazz.newInstance();
setAge.invoke(person, 89);
System.out.println(person.getAge());
/**3.属性**/
//获取当前类的属性，不包括父类
Field[] fields = clazz.getDeclaredFields();
for (Field field : fields) {
System.out.println(field.getName());
}
//获取当前类的指定属性
Field name = clazz.getDeclaredField("name");
System.out.println(name.getName());
//获取属性的值
Person person1 = new Person("Maria", 7);
String s = (String) name.get(person1);
System.out.println(s);
//设置对象的值
Person person2 = new Person();
Field age = clazz.getDeclaredField("age");
age.setAccessible(true); //私有属性，设置 setAccessible(true)
age.set(person2, 5);
System.out.println(person2.getAge());
//获取当前类的指定私有属性
Field age2 = clazz.getDeclaredField("age");
age2.setAccessible(true);
System.out.println(age2.getName());
}
}为其他对象提供一种代理以控制对这个对象的访问（代理类相当于一个中介）。
//ISeller.java
public interface ISeller {
void sell();
}
//Factory.java
public class Factory implements ISeller {
@Override
public void sell() {
System.out.println("厂家直销");
}
}
//Daigou.java
public class Daigou implements ISeller {
private ISeller seller;
public Daigou(ISeller seller) {
this.seller = seller;
}
@Override
public void buy() {
doBefore();
//真正调用的持有的类的方法
mSeller.sell();
doAfter();
}
private void doBefore() {
System.out.println("加价899");
}
private void doAfter() {
System.out.println("提供售后");
}
}
//TestProxy.java
public class TestProxy {
public static void main(String[] args) {
Factory factory = new Factory();
Daigou daigou = new Daigou(factory);
daigou.buy();
}
}
//假如我们买一个面膜，找到代购，代购其实也是别处买的，代购加价 899 元卖给你，我们不直接和厂家发生关系，这种就是一个代理模型代理模式可以在不修改被代理对象的基础上，通过扩展代理类，进行一些功能的附加与增强。值得注意的是，代理类和被代理类应该共同实现一个接口，或者是共同继承某个类。
但是有弊端，假如这人不仅不仅要面膜，还要核导弹、航空母舰等等，就创建了许多代理类。
//ISeller.java
public interface ISeller {
void sell();
}
//SuperDaigou.java
public class SuperDaigou implements ISeller {
@Override
public void sell() {
System.out.println("超级代购");
}
}
//ProxyHandler.java
public class ProxyHandler implements InvocationHandler {
//声明目标对象
private ISeller target;
public ProxyHandler(ISeller target) {
this.target = target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
doBefore();
Object invoke = method.invoke(target, args);
doAfter();
return invoke;
}
//得到代理对象
public Object getProxyInstance() {
return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this);
}
private void doBefore() {
System.out.println("加价899");
}
private void doAfter() {
System.out.println("提供售后");
}
}
public class TestProxy {
public static void main(String[] args) {
SuperDaigou superDaigou = new SuperDaigou();
ProxyHandler handler = new ProxyHandler(superDaigou);
//增强原来的方法
ISeller seller = (ISeller) handler.getProxyInstance();
seller.sell();
}
}由于使用了反射，效率比较低。
//Proxy.java
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
//1.动态生成 class 文件字节流，然后通过 loader 加载此字节流创建代理类 class
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//2.获取代理类的类构造对象
return cons.newInstance(new Object[]{h});
} catch (IllegalAccessException|InstantiationException e) {
throw new InternalError(e.toString(), e);
} catch (InvocationTargetException e) {
Throwable t = e.getCause();
if (t instanceof RuntimeException) {
throw (RuntimeException) t;
} else {
throw new InternalError(t.toString(), t);
}
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString(), e);
}
}
//Proxy.java
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// If the proxy class defined by the given loader implementing
// the given interfaces exists, this will simply return the cached copy;
// otherwise, it will create the proxy class via the ProxyClassFactory
return proxyClassCache.get(loader, interfaces);
}
// WeakCache.java
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
Object cacheKey = CacheKey.valueOf(key, refQueue);
// 根据cachekey获取键值对valuesMap, valuesMap的key是接口列表的包装类，value是动态生成代理类的包装类
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
ConcurrentMap<Object, Supplier<V>> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}
// create subKey and retrieve the possible Supplier<V> stored by that
//核心：获取代理类 ProxyClassFactory#apply
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
V value = supplier.get();
if (value != null) {
return value;
}
}
// else no supplier in cache
// or a supplier that returned null (could be a cleared CacheValue
// or a Factory that wasn't successful in installing the CacheValue)
// 如果动态生成代理类的工厂类为空，则创建新的工厂类
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
//工厂类的包装类为空，则创建新的包装类supplier
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
// successfully installed Factory
supplier = factory;
}
// else retry with winning supplier
} else {
if (valuesMap.replace(subKey, supplier, factory)) {
// successfully replaced
// cleared CacheEntry / unsuccessful Factory
// with our Factory
supplier = factory;
} else {
// retry with current supplier
supplier = valuesMap.get(subKey);
}
}
}
}
//以上代码：通过 map 来存储动态生成的代理类，其中 key 是接口的包装类，value 是动态代理类的包装类
//Proxy.java#ProxyClassFactory
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class<?> interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null; // package to define proxy class in
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
/*
* Record the package of a non-public proxy interface so that the
* proxy class will be defined in the same package. Verify that
* all non-public proxy interfaces are in the same package.
*/
for (Class<?> intf : interfaces) {
int flags = intf.getModifiers();
if (!Modifier.isPublic(flags)) {
accessFlags = Modifier.FINAL;
String name = intf.getName();
int n = name.lastIndexOf('.');
String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
if (proxyPkg == null) {
proxyPkg = pkg;
} else if (!pkg.equals(proxyPkg)) {
throw new IllegalArgumentException(
"non-public interfaces from different packages");
}
}
}
if (proxyPkg == null) {
// if no non-public proxy interfaces, use com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
//代理类名称
String proxyName = proxyPkg + proxyClassNamePrefix + num;
//代理类字节码
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//最终生成代理类的 class 对象是本地方法 defineClass0 方法
//原理是根据类名、接口、类加载器、方法列表、异常列表，按照 class 文件格式先生成字节流，再生成动态代理类
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
/*
* A ClassFormatError here means that (barring bugs in the
* proxy class generation code) there was some other
* invalid aspect of the arguments supplied to the proxy
* class creation (such as virtual machine limitations
* exceeded).
*/
throw new IllegalArgumentException(e.toString());
}
}
}