1.java.reflection
2.静态语言VS动态语言
参数,方法用private修饰时,一般只能有get/set去获取或者设定值,但是反射可以直接去获取到private的参数值
3.java反射机制提供的功能
4.反射的优点和缺点
5.反射相关的主要Api
代码:
package com.qiu.reflection;
//什么叫反射
public class Test02 {
public static void main(String[] args) throws ClassNotFoundException {
Class c1 = Class.forName("com.qiu.reflection.User");
Class c2 = Class.forName("com.qiu.reflection.User");
Class c3 = Class.forName("com.qiu.reflection.User");
Class c4 = Class.forName("com.qiu.reflection.User");
//一个类在内存中只有一个class对象
//一个类被加载后,类的整个结构,都会被封装在Class对象中 System.out.println(c1.hashCode());
System.out.println(c2.hashCode());
System.out.println(c3.hashCode());
System.out.println(c4.hashCode());
}
}
//实体类:pojo,entity
class User{
private String name;
private int id ;
private int age; public User() {
} public User(String name, int id, int age) {
this.name = name;
this.id = id;
this.age = age;
} public String getName() {
return name;
} public void setName(String name) {
this.name = name;
} public int getId() {
return id;
} public void setId(int id) {
this.id = id;
} public int getAge() {
return age;
} public void setAge(int age) {
this.age = age;
} @Override
public String toString() {
return "User{" +
"name='" + name + ''' +
", id=" + id +
", age=" + age +
'}';
}
}
6.Class类
7.Class类的常用方法
8.获取Class类的实例
9.哪些类可以有class对象
package com.qiu.reflection;import java.lang.annotation.ElementType;//所有类型的Class
public class Test04 {
public static void main(String[] args) {
Class c1 = Object.class;//类
Class c2 = Comparable.class;//接口
Class c3 = String[].class;//数组
Class c4 = int.class;//二维数组
Class c5 = Override.class;//注解
Class c6 = ElementType.class;//枚举
Class c7 = Integer.class;//基本数据类型
Class c8 = void.class;//void
Class c9 = Class.class;//Class
System.out.println(c1);
System.out.println(c2);
System.out.println(c3);
System.out.println(c4);
System.out.println(c5);
System.out.println(c6);
System.out.println(c7);
System.out.println(c8);
System.out.println(c9);
//只要元素类型和维度一样,就是同一个Class
int[] a =new int[10];
int[] b =new int[100];
System.out.println(a.getClass().hashCode());
System.out.println(b.getClass().hashCode()); }
}
10.java内存分析
11.什么时候类会发生初始化
package com.qiu.reflection;//测试类什么时候会初始化
public class Test06 {
static {
System.out.println("main被加载");
} public static void main(String[] args) throws ClassNotFoundException {
//主动引用
Son son = new Son();
//反射也会产生主动引用
Class.forName("com.qiu.reflection.Son");
//不会产生类的引用的方法
System.out.println(Son.b);//当访问一个静态域时,只有真正声明这个类才会被初始化.如:当通过子类引用父类的的静态变量,不会导致子类初始化
Son[] array = new Son[5];//通过数组定义类,不会触发此类的初始化 System.out.println(Son.M);//引用常量不会触发此类的初始化
}
}
class Father{
static int b=2
static {
System.out.println("父类被加载");
}
}
class Son extends Father{
static {
System.out.println("子类被加载");
m=300;
}
static int m =100;
static final int M=1;
}
12.类加载器的作用
package com.qiu.reflection;public class Test07 {
public static void main(String[] args) throws ClassNotFoundException {
ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();//获取系统类的加载器
System.out.println(systemClassLoader); //获取系统加载器的父类加载器-->扩展类加载器 ClassLoader parent = systemClassLoader.getParent();
System.out.println(parent); //获取扩展类加载器的父类加载器-->根加载器(c/c++) ClassLoader parent1 = parent.getParent();
System.out.println(parent1); //测试当前类是哪个加载器加载的
ClassLoader classLoader = Class.forName("com.qiu.reflection.Test07").getClassLoader();//自定义的类,是系统类加载器加载的
System.out.println(classLoader);
//测试JDK内置类是哪个加载器加载的
classLoader = Class.forName("java.lang.Object").getClassLoader();//由于JDK内置的类是引导类加载器,也就是根加载器加载的所以无法获取
System.out.println(classLoader); //测试获得系统加载器可以加载的路径
String property = System.getProperty("java.class.path");
System.out.println(property);
/*
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\charsets.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\deploy.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\access-bridge-64.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\cldrdata.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\dnsns.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\jaccess.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\jfxrt.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\localedata.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\nashorn.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\sunec.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\sunjce_provider.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\sunmscapi.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\sunpkcs11.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\ext\zipfs.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\javaws.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\jce.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\jfr.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\jfxswt.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\jsse.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\management-agent.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\plugin.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\resources.jar;
C:\Users\qiuzhikang\Desktop\jdk1.8.0_162\jre\lib\rt.jar;
E:\mix_email\java_thread\out\production\annotation;
E:\Idea 2019\IntelliJ IDEA 2019.3.1\lib\idea_rt.jarProcess finished with exit code 0 */
}
}
13.创建运行时的类
package com.qiu.reflection;import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;public class Test08 {
public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
//1.
Class c1 = Class.forName("com.qiu.reflection.User");
//2.
User user = new User();
c1= user.getClass();
//获得类的名字
System.out.println(c1.getName());//获得包名加类名
System.out.println(c1.getSimpleName());//获得类名
//获得类的属性
System.out.println("========");
Field[] field = c1.getFields();//只能找到public属性
/*
for (Field field1 : field) {
System.out.println(field1);
}
获取不出来
*/
field = c1.getDeclaredFields();//找到全部得属性
for (Field field1 : field) {
System.out.println(field1);
}
//获得指定属性的值
Field name = c1.getDeclaredField("name");
System.out.println(name);
//获得类的方法
System.out.println("====");
Method[] methods = c1.getMethods();//获得本类及其父类的全部public方法
for (Method method : methods) {
System.out.println("正常的"+method);
}
System.out.println("=====");
methods = c1.getDeclaredMethods();//获得本类的全部方法(包括private方法),无法获取父类的方法
for (Method method : methods) {
System.out.println(method);
}
//获得指定的方法
System.out.println("==============");
Method getName = c1.getMethod("getName", null);
Method setName = c1.getMethod("setName", String.class);
System.out.println(getName);
System.out.println(setName); //获得指定的构造器 System.out.println("=========+");
Constructor[] constructor = c1.getConstructors();
Constructor[] constructor1 = c1.getDeclaredConstructors();
for (Constructor constructor2 : constructor) {
System.out.println(constructor2);
}
for (Constructor constructor2 : constructor1) {
System.out.println("::"+constructor2);
}
Constructor constructor2 = c1.getConstructor(String.class, int.class, int.class);
System.out.println("指定的:"+constructor2);
}
}
package com.qiu.reflection;import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;//通过反射,动态的创建对象
public class Test09 {
public static void main(String[] args) throws Exception {
//获得Class对象
Class c1= Class.forName("com.qiu.reflection.User");
//构造一个对象
User user = (User) c1.newInstance();//本质上是调用一个无参构造器
System.out.println(user);
//如果实体类没有无参构造器,那就可以用以下步骤(**)
//通过构造器创建对象
Constructor declaredConstructor = c1.getDeclaredConstructor(String.class, int.class, int.class);(**)
User user2 = (User)declaredConstructor.newInstance("qiuzhikang", 001, 18);
System.out.println(user2); //通过反射调用普通方法
User user3 = (User) c1.newInstance();
//通过反射获取一个方法
Method setName = c1.getDeclaredMethod("setName", String.class);
//invoke :激活的意思
//(对象,"方法值)
setName.invoke(user3, "我");
System.out.println(user3.getName()); //通过反射操作属性
User user4 = (User) c1.newInstance();
Field name = c1.getDeclaredField("name");
//不能直接操作私有属性,我们需要关闭安全监测,属性或者方法的setAccessible(true)
name.setAccessible(true);//默认为false,表示安全监测开启
name.set(user4,"你");
System.out.println(user4.getName());
}
}
14性能对比分析
package com.qiu.reflection;import java.lang.reflect.Method;//分析性能问题
public class Test10 { //普通方式调用
public static void test01(){
User user = new User();
Long startTime = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
user.getName();
}
Long endTime = System.currentTimeMillis();
System.out.println("普通方法执行10亿次:"+(endTime-startTime)+"ms");
} //反射方式调用
public static void test02() throws Exception {
User user = new User();
Class c1 = user.getClass(); Method getName = c1.getDeclaredMethod("getName", null); Long startTime = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
getName.invoke(user,null);
}
Long endTime = System.currentTimeMillis();
System.out.println("反射方法执行10亿次:"+(endTime-startTime)+"ms"); } //反射方式调用,关闭检测
public static void test03() throws Exception {
User user = new User();
Class c1 = user.getClass(); Method getName = c1.getDeclaredMethod("getName", null);
getName.setAccessible(true);
Long startTime = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
getName.invoke(user,null);
}
Long endTime = System.currentTimeMillis();
System.out.println("关闭检测后反射方法执行10亿次:"+(endTime-startTime)+"ms"); } public static void main(String[] args) throws Exception {
test01();
test02();
test03();
}}
15.获取泛型信息
package com.qiu.reflection;import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;//通过反射获取泛型
public class Test11 {
//没有返回值的泛型
public void test01(Map<String,User> map, List<User> list){
System.out.println("test01");
}
//有返回值的泛型
public Map<String,User> test02(){
System.out.println("test02");
return null;
}
//
public static void main(String[] args) throws Exception {
Method method = Test11.class.getMethod("test01", Map.class, List.class);
Type[] genericParameterTypes = method.getGenericParameterTypes();//获得泛型的参数类型
for (Type genericParameterType : genericParameterTypes) {
System.out.println("#" + genericParameterType);
if (genericParameterType instanceof ParameterizedType) {//判断是不是一个参数化类型
Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments();
for (Type actualTypeArgument : actualTypeArguments) {
System.out.println(actualTypeArgument);
}
}
}
method = Test11.class.getMethod("test02", null);
Type genericReturnType = method.getGenericReturnType();
if (genericReturnType instanceof ParameterizedType) {
Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();
for (Type actualTypeArgument : actualTypeArguments) {
System.out.println(actualTypeArgument);
}
} }
}
16.通过反射来操作注解
package com.qiu.reflection;import java.lang.annotation.*;
import java.lang.reflect.Field;
import java.nio.channels.ClosedSelectorException;//练习反射操作注解
public class Test12 {
public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
Class c1 = Class.forName("com.qiu.reflection.Stdent2"); //通过反射获得注解
Annotation[] annotations = c1.getAnnotations();
for (Annotation annotation : annotations) {
System.out.println(annotation);
}
//获得注解Value的值
TableQiu Tableqiu =(TableQiu) c1.getAnnotation(TableQiu.class);
String value = Tableqiu.value();
System.out.println(value); //获得类指定的注解
Field f = c1.getDeclaredField("name");
FieldQiu annotation = f.getAnnotation(FieldQiu.class);
System.out.println(annotation.columnName());
System.out.println(annotation.length());
System.out.println(annotation.type()); }}
@TableQiu("db_student")
class Stdent2{
@FieldQiu(columnName = "db_id",type ="int",length = 10)
private int id;
@FieldQiu (columnName = "db_age",type ="int",length = 10)
private int age;
@FieldQiu(columnName = "db_name",type ="varchar",length = 10)
private String name; public Stdent2() {
} public Stdent2(int id, int age, String name) {
this.id = id;
this.age = age;
this.name = name;
} public int getId() {
return id;
} public void setId(int id) {
this.id = id;
} public int getAge() {
return age;
} public void setAge(int age) {
this.age = age;
} public String getName() {
return name;
} public void setName(String name) {
this.name = name;
} @Override
public String toString() {
return "Stdent2{" +
"id=" + id +
", age=" + age +
", name='" + name + ''' +
'}';
}
}@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableQiu{
String value();
}@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldQiu{
String columnName();
String type();
int length();
}
自己手敲的源代码,放在了GItee上面,放个链接:
https://gitee.com/qiu71953689/java_annotation_reflection_code