JAVA反射

参考：https://www.liaoxuefeng.com/wiki/1252599548343744/1255945147512512

基本逻辑

package reflection;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
 * 教程：https://www.liaoxuefeng.com/wiki/1252599548343744/1264799402020448
 * class是由JVM在执行过程中动态加载的。JVM在第一次读取到一种class类型时，将其加载进内存。
 * 每加载一种class，JVM就为其创建一个Class类型的实例，并关联起来。注意：这里的Class类型是一个名叫Class的class。
 */
public class MainTest2 {

    public static void main(String[] args) {
        /**
         * 由于JVM为每个加载的class创建了对应的Class实例，并在实例中保存了该class的所有信息，包括类名、包名、父类、实现的接口、
         * 所有方法、字段等，因此，如果获取了某个Class实例，我们就可以通过这个Class实例获取到该实例对应的class的所有信息。
         *
         * 这种通过Class实例获取class信息的方法称为反射（Reflection）。
         *
         * 如何获取一个class的Class实例？有三个方法：
         */


        // 方法1
        Class cls1 = String.class;

        // 方法2
        String s = "Hello";
        Class cls2 = s.getClass();  // 通过实例变量的getClass()获得

        // 方法3
        try {
            Class cls3 = Class.forName("java.lang.String");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

        // 因为Class实例在JVM中是唯一的，所以，上述方法获取的Class实例是同一个实例。可以用==比较两个Class实例：


        /**通过Class实例获取相关信息**/
        Class clsStu = Student.class;

        System.out.println(clsStu.getClassLoader());
        System.out.println(clsStu.getName());
        try {
            System.out.println(clsStu.getConstructor());
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
        System.out.println(clsStu.getDeclaredFields().toString());
        System.out.println(clsStu.getFields().toString());
        try {
            System.out.println(clsStu.getField("grade"));
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        try {
            System.out.println(clsStu.getField("score"));
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        try {
            System.out.println(clsStu.getField("name"));
            Field field = clsStu.getField("name");
            System.out.println("type=" + field.getType() + ", name=" + field.getName() + ", ");
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }



        try {

            /**通过Class实例调用方法**/
            String str = "helloWorld";
            Class clsz1 = str.getClass();
            Class clsz2 = String.class;
            Class clsz3 = Class.forName("java.lang.String");

            System.out.println("1Equal2:" + clsz1.equals(clsz2) + ", 1Equal3:" + clsz1.equals(clsz3));  // true  true
            System.out.println("1==2:" + (clsz1 == clsz2) + ", 1==3:" + (clsz1 == clsz3));  // true  true

            Method method = clsz2.getMethod("substring", int.class, int.class);
            System.out.println("substring=" + method.invoke(str, 0, 3));



            Method m = Integer.class.getMethod("parseInt", String.class);
            // 调用该静态方法parseInt，无需传入对象（传null即可）
            Integer n = (Integer) m.invoke(null, "12345");
            System.out.println("n=" + n);


            /**通过Class实例调用构造方法**/
            Person p1 = new Person();
            Person p2 = Person.class.newInstance();  // 调用Class.newInstance()的局限是，它只能调用该类的public无参数构造方法
            System.out.println("p1.equals.p2:"  + p1.equals(p2));  // false
            System.out.println("p1==p2:"  + p1.equals(p2));  // false

            // 使用 Java的反射API提供的Constructor对象
            Constructor constructor1 = Integer.class.getConstructor(String.class);  // 获取Integer参数类型为String的构造方法
            int resInt = (int) constructor1.newInstance("987");
            System.out.println("resInt=" + resInt);



        } catch (Exception e) {
            e.printStackTrace();
        }


    }

    static class Person{
        public String name;
    }

    class Student extends Person {
        public int score;
        private int grade;
    }
}

执行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/bin/java -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/lib/tools.jar:/Users/wuxiong.wx/work/code/testDemo/target/classes:/Users/wuxiong.wx/.m2/repository/org/springframework/spring/999-not-exist/spring-999-not-exist.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-core/4.3.2.RELEASE/spring-core-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/commons-logging/commons-logging/1.2/commons-logging-1.2.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-beans/4.3.2.RELEASE/spring-beans-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-aop/4.3.2.RELEASE/spring-aop-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-context/4.3.2.RELEASE/spring-context-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-expression/4.3.2.RELEASE/spring-expression-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-context-support/4.3.2.RELEASE/spring-context-support-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-tx/4.3.2.RELEASE/spring-tx-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-jdbc/4.3.2.RELEASE/spring-jdbc-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-orm/4.3.2.RELEASE/spring-orm-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-web/4.3.2.RELEASE/spring-web-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-webmvc/4.3.2.RELEASE/spring-webmvc-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/springframework/spring-test/4.3.2.RELEASE/spring-test-4.3.2.RELEASE.jar:/Users/wuxiong.wx/.m2/repository/org/aspectj/aspectjrt/1.8.8/aspectjrt-1.8.8.jar:/Users/wuxiong.wx/.m2/repository/org/aspectj/aspectjweaver/1.8.8/aspectjweaver-1.8.8.jar:/Users/wuxiong.wx/.m2/repository/junit/junit/4.12/junit-4.12.jar:/Users/wuxiong.wx/.m2/repository/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar:/Users/wuxiong.wx/.m2/repository/io/appium/java-client/7.0.0/java-client-7.0.0.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-java/3.141.59/selenium-java-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-chrome-driver/3.141.59/selenium-chrome-driver-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-edge-driver/3.141.59/selenium-edge-driver-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-firefox-driver/3.141.59/selenium-firefox-driver-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-ie-driver/3.141.59/selenium-ie-driver-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-opera-driver/3.141.59/selenium-opera-driver-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-remote-driver/3.141.59/selenium-remote-driver-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-safari-driver/3.141.59/selenium-safari-driver-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/net/bytebuddy/byte-buddy/1.8.15/byte-buddy-1.8.15.jar:/Users/wuxiong.wx/.m2/repository/org/apache/commons/commons-exec/1.3/commons-exec-1.3.jar:/Users/wuxiong.wx/.m2/repository/com/google/guava/guava/25.0-jre/guava-25.0-jre.jar:/Users/wuxiong.wx/.m2/repository/com/google/code/findbugs/jsr305/1.3.9/jsr305-1.3.9.jar:/Users/wuxiong.wx/.m2/repository/org/checkerframework/checker-compat-qual/2.0.0/checker-compat-qual-2.0.0.jar:/Users/wuxiong.wx/.m2/repository/com/google/errorprone/error_prone_annotations/2.1.3/error_prone_annotations-2.1.3.jar:/Users/wuxiong.wx/.m2/repository/com/google/j2objc/j2objc-annotations/1.1/j2objc-annotations-1.1.jar:/Users/wuxiong.wx/.m2/repository/org/codehaus/mojo/animal-sniffer-annotations/1.14/animal-sniffer-annotations-1.14.jar:/Users/wuxiong.wx/.m2/repository/com/squareup/okhttp3/okhttp/3.11.0/okhttp-3.11.0.jar:/Users/wuxiong.wx/.m2/repository/com/squareup/okio/okio/1.14.0/okio-1.14.0.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-support/3.141.59/selenium-support-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/org/seleniumhq/selenium/selenium-api/3.141.59/selenium-api-3.141.59.jar:/Users/wuxiong.wx/.m2/repository/com/google/code/gson/gson/2.8.5/gson-2.8.5.jar:/Users/wuxiong.wx/.m2/repository/org/apache/httpcomponents/httpclient/4.5.6/httpclient-4.5.6.jar:/Users/wuxiong.wx/.m2/repository/org/apache/httpcomponents/httpcore/4.4.10/httpcore-4.4.10.jar:/Users/wuxiong.wx/.m2/repository/commons-codec/commons-codec/1.10/commons-codec-1.10.jar:/Users/wuxiong.wx/.m2/repository/cglib/cglib/3.2.8/cglib-3.2.8.jar:/Users/wuxiong.wx/.m2/repository/org/ow2/asm/asm/6.2.1/asm-6.2.1.jar:/Users/wuxiong.wx/.m2/repository/org/apache/ant/ant/1.10.3/ant-1.10.3.jar:/Users/wuxiong.wx/.m2/repository/org/apache/ant/ant-launcher/1.10.3/ant-launcher-1.10.3.jar:/Users/wuxiong.wx/.m2/repository/commons-validator/commons-validator/1.6/commons-validator-1.6.jar:/Users/wuxiong.wx/.m2/repository/commons-beanutils/commons-beanutils/1.9.2/commons-beanutils-1.9.2.jar:/Users/wuxiong.wx/.m2/repository/commons-digester/commons-digester/1.8.1/commons-digester-1.8.1.jar:/Users/wuxiong.wx/.m2/repository/commons-collections/commons-collections/3.2.2/commons-collections-3.2.2.jar:/Users/wuxiong.wx/.m2/repository/org/apache/commons/commons-lang3/3.8/commons-lang3-3.8.jar:/Users/wuxiong.wx/.m2/repository/commons-io/commons-io/2.6/commons-io-2.6.jar:/Users/wuxiong.wx/.m2/repository/org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar:/Users/wuxiong.wx/.m2/repository/com/alibaba/fastjson/1.2.60/fastjson-1.2.60.jar reflection.MainTest2
sun.misc.Launcher$AppClassLoader@135fbaa4
reflection.MainTest2$Student
[Ljava.lang.reflect.Field;@7ea987ac
[Ljava.lang.reflect.Field;@5cad8086
public int reflection.MainTest2$Student.score
public java.lang.String reflection.MainTest2$Person.name
type=class java.lang.String, name=name, 
1Equal2:true, 1Equal3:true
1==2:true, 1==3:true
substring=hel
n=12345
p1.equals.p2:false
p1==p2:false
resInt=987
java.lang.NoSuchMethodException: reflection.MainTest2$Student.<init>()
    at java.lang.Class.getConstructor0(Class.java:3082)
    at java.lang.Class.getConstructor(Class.java:1825)
    at reflection.MainTest2.main(MainTest2.java:48)
java.lang.NoSuchFieldException: grade
    at java.lang.Class.getField(Class.java:1703)
    at reflection.MainTest2.main(MainTest2.java:55)

Process finished with exit code 0

动态代理

https://www.liaoxuefeng.com/wiki/1252599548343744/1264804593397984

另外的总结：https://www.zhihu.com/question/20794107/answer/604485904

有没有可能不编写实现类，直接在运行期创建某个interface的实例呢？

这是可能的，因为Java标准库提供了一种动态代理（Dynamic Proxy）的机制：可以在运行期动态创建某个interface的实例。

什么叫运行期动态创建？听起来好像很复杂。所谓动态代理，是和静态相对应的。我们来看静态代码怎么写：

定义接口：

public interface Hello {
    void morning(String name);
}

编写实现类：

public class HelloWorld implements Hello {
    public void morning(String name) {
        System.out.println("Good morning, " + name);
    }
}

创建实例，转型为接口并调用：

Hello hello = new HelloWorld();
hello.morning("Bob");

还有一种方式是动态代码，我们仍然先定义了接口Hello，但是我们并不去编写实现类，而是直接通过JDK提供的一个Proxy.newProxyInstance()创建了一个Hello接口对象。这种没有实现类但是在运行期动态创建了一个接口对象的方式，我们称为动态代码。JDK提供的动态创建接口对象的方式，就叫动态代理。

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class Main {
    public static void main(String[] args) {
        InvocationHandler handler = new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                System.out.println(method);
                if (method.getName().equals("morning")) {
                    System.out.println("Good morning, " + args[0]);
                }
                return null;
            }
        };
        Hello hello = (Hello) Proxy.newProxyInstance(
            Hello.class.getClassLoader(), // 传入ClassLoader
            new Class[] { Hello.class }, // 传入要实现的接口
            handler); // 传入处理调用方法的InvocationHandler
        hello.morning("Bob");
    }
}

interface Hello {
    void morning(String name);
}

public abstract void Hello.morning(java.lang.String)
Good morning, Bob

例：

package designpattern.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class DynamicProxyTest {

    public static void main(String[] args) {
        AbstractSubject abstractSubject = null;
        InvocationHandler invocationHandler = null;

        // jdk动态代理 设置此系统属性,让JVM生成的Proxy类写入文件
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

        invocationHandler = new DynamicProxy(new RealSubjectA());
        abstractSubject = (AbstractSubject) Proxy.newProxyInstance(
                AbstractSubject.class.getClassLoader(),
                new Class[] {AbstractSubject.class},
                invocationHandler);

        abstractSubject.request();

    }


}

interface AbstractSubject {
    void request();
}


class RealSubjectA implements AbstractSubject {

    public void request() {
        System.out.println("访问真实主题A");
    }
}

class RealSubjectB implements AbstractSubject {

    public void request() {
        System.out.println("访问真实主题B");
    }
}


/**
 * 代理类主要负责为委托了（真实对象）预处理消息、过滤消息、传递消息给委托类，
 * 代理类不现实具体服务，而是利用委托类来完成服务，并将执行结果封装处理。
 */
class DynamicProxy implements InvocationHandler {

    Object object;

    public DynamicProxy(Object object) {
        this.object = object;
    }

    public Object invoke(Object proxy, Method m, Object[] args) {
        try {
            beforeRequest();
            m.invoke(object, args);  // 这里第一个参数是object，而不是p
            afterRequest();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }

        return null;
    }

    void beforeRequest() {
        System.out.println("预处理");
    }

    void afterRequest() {
        System.out.println("后处理");
    }
}

预处理
访问真实主题A
后处理