# 什么是反射机制
运行状态中，对于任意一个类，都能够知道这个类的所有属性和方法。对于任意一个对象，都能够调用它的任意属性和方法。动态获取java类信息，以及动态调用对象方法的功能，便是反射机制。
# 反射机制
# 反射机制的相关类及抽象
- 类,Lang包
public final class Class<T> implements java.io.Serializable,
GenericDeclaration,
Type,
AnnotatedElement {
//创建类实例
@CallerSensitive
public T newInstance()
throws InstantiationException, IllegalAccessException;
//获取包名
public Package getPackage();
//获取类属性
public Field[] getDeclaredFields() throws SecurityException ;
//获取类方法
@CallerSensitive
public Method[] getDeclaredMethods() throws SecurityException;
//获取构造器
@CallerSensitive
public Constructor<?>[] getDeclaredConstructors() throws SecurityException;
//获取构造器
public Annotation[] getDeclaredAnnotations();
//是否是注解类
boolean isAnnotation();
//是否是匿名类
boolean isAnonymousClass();
//是否是接口
boolean isInterface();
//是否是内部类
boolean isMemberClass();
//省略其它抽象...
}
- 属性类Field,reflect包
public final
class Field extends AccessibleObject implements Member {
//获取属性值
@CallerSensitive
public Object get(Object obj);
//设置属性值
@CallerSensitive
public void set(Object obj, Object value)
throws IllegalArgumentException, IllegalAccessException;
//省略其它抽象...
}
- 方法类Method,reflect包
public final class Method extends Executable {
//动态执行方法
@CallerSensitive
public Object invoke(Object obj, Object... args)
throws IllegalAccessException, IllegalArgumentException,
InvocationTargetException;
//省略其它抽象...
}
- 构造器类Constructor,reflect包
public final class Constructor<T> extends Executable{
//实例化对象
@CallerSensitive
public T newInstance(Object ... initargs)
throws InstantiationException, IllegalAccessException,
IllegalArgumentException, InvocationTargetException;
//省略其它抽象...
}
# 反射机制的优缺点
- 优点: 动态加载类，提高灵活度
- 缺点:
- 性能瓶颈
- 安全问题
# 代理模式
# 静态代理
- 对目标对象的增强，通过手动完成;
# 动态代理
- JDK动态代理
- 组成：
- Proxy: newProxyInstance对象
- InvocationHandler: invoke方法,实现对目标对象的增强
- 问题: 只能代理接口
- 组成：
- CGLIB动态代理
- 组成:
- Enhancer: create方法
- MethodInterceptor: intercept方法，实现对目标对象的增强
- 组成:
# JDK动态代理原理
- 结论: 是动态创建代理类井通过指定类加载器加载，然后在创建代理对象时将InvokerHandler对象作为构造参数传入
public class Proxy implements java.io.Serializable {
//精简版的动态代理方法
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
Class<?> cl = getProxyClass0(loader, intfs);
try {
final Constructor<?> cons = cl.getConstructor({ InvocationHandler.class });
return cons.newInstance(new Object[]{h});
} catch (Exception e) {
//do nothing
}
}
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
return proxyClassCache.get(loader, interfaces);
}
}
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
@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();
String proxyName = proxyPkg + "$Proxy" + num;
/*
* Generate the specified proxy class.
*/
/*xxx: ProxyGenerator.generateProxyClass（） 方法会按照指定 名称和接口集合生成代理类的字节
码，并根据条件决定是否保存到磁盘上*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
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());
}
}
}
public class ProxyGenerator {
/*xxx: 生成字节码*/
public static byte[] generateProxyClass(final String clazzName, Class<?>[] interfaces, int accessFlags) {
ProxyGenerator generator = new ProxyGenerator(clazzName, interfaces, accessFlags);
final byte[] bytecodes = generator.generateClassFile();
if (saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
try {
int var1 = clazzName.lastIndexOf(46);
Path path;
if (var1 > 0) {
Path var3 = Paths.get(clazzName.substring(0, var1).replace('.', File.separatorChar));
Files.createDirectories(var3);
path = var3.resolve(clazzName.substring(var1 + 1, clazzName.length()) + ".class");
} else {
path = Paths.get(clazzName + ".class");
}
//根据条件，是否保存至文件中
Files.write(path, bytecodes, new OpenOption[0]);
return null;
} catch (IOException var4x) {
throw new InternalError("I/O exception saving generated file: " + var4x);
}
}
});
}
return bytecodes;
}
private byte[] generateClassFile() {
//省略....
if (this.methods.size() > 65535) {
throw new IllegalArgumentException("method limit exceeded");
} else if (this.fields.size() > 65535) {
throw new IllegalArgumentException("field limit exceeded");
} else {
this.cp.getClass(dotToSlash(this.className));
this.cp.getClass("java/lang/reflect/Proxy");
interfaceArray = this.interfaces;
arrayLength = interfaceArray.length;
for(index = 0; index < arrayLength; ++index) {
clazz = interfaceArray[index];
this.cp.getClass(dotToSlash(clazz.getName()));
}
this.cp.setReadOnly();
ByteArrayOutputStream arrayOutputStream = new ByteArrayOutputStream();
DataOutputStream dataOutputStream = new DataOutputStream(arrayOutputStream);
try {
dataOutputStream.writeInt(-889275714);
dataOutputStream.writeShort(0);
dataOutputStream.writeShort(49);
this.cp.write(dataOutputStream);
dataOutputStream.writeShort(this.accessFlags);
dataOutputStream.writeShort(this.cp.getClass(dotToSlash(this.className)));
dataOutputStream.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
dataOutputStream.writeShort(this.interfaces.length);
Class[] var17 = this.interfaces;
int var18 = var17.length;
for(int var19 = 0; var19 < var18; ++var19) {
Class var22 = var17[var19];
dataOutputStream.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
}
dataOutputStream.writeShort(this.fields.size());
var15 = this.fields.iterator();
while(var15.hasNext()) {
ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
var20.write(dataOutputStream);
}
dataOutputStream.writeShort(this.methods.size());
var15 = this.methods.iterator();
while(var15.hasNext()) {
ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
var21.write(dataOutputStream);
}
dataOutputStream.writeShort(0);
/*xxx: 通过 DataOutputSteam 返回字节码*/
return arrayOutputStream.toByteArray();
} catch (IOException var9) {
throw new InternalError("unexpected I/O Exception", var9);
}
}
}
}
- 一个反编译后的动态代理类:
interface CustomInterface{
void doSomeThing();
}
public final class $Proxy1 extends Proxy implements CustomInterface{
private static Method m3;
static {
m3 = Class.forName("xxx.xxx.CustomInterface").getMethod("doSomeThing",new Class[0]);
}
public $Proxy1(InvocationHandler h) throws Exception{
super(h);
}
public final void doSomeThing() throws Exception{
super.h.invoke(this,m3,(Object[])null);
}
}
# 反射注意事项
# getFields(),getMethods()反射的局限性
- 只能获取public类型的属性，或者方法;
# getDeclaredFields(),getDeclaredMethods()
- 既能获取到public类型的成员，又能获取private的成员
- 对于private的成员，需要先设置 setAccessible