反射的實(shí)現(xiàn)原理

  • 反射的demo

    package com.java.reflect;
import java.lang.reflect.Method;
/**
 * 反射調(diào)用的例子
 * @author helenlee
 * @date 2018/9/4
 */
public class ReflectionDemo {
    public static void main(String[] args) throws Exception {
        Class<?> clzss = Class.forName("com.java.reflect.ReflectionDemo");
        // target方法是訪問(wèn)權(quán)限是public，
        // 如果是private則需要通過(guò)clzss.getDeclaredMethod()獲取
        Method target = clzss.getMethod("target", int.class);
        target.invoke(null, 128);
    }

    public static void target(int i) {
        System.out.println("invoke target method");
    }
}
###輸出
$ javac com/java/reflect/ReflectionDemo.java 
$ java com/java/reflect/ReflectionDemo
invoke target method

  • 反射的方法的執(zhí)行鏈路

    為了看到反射調(diào)用鏈路我們以構(gòu)造異常的方式打印堆棧信息：

    package com.java.reflect;
import java.lang.reflect.Method;
/**
 * 反射調(diào)用的例子
 * @author helenlee
 * @date 2018/9/4
 */
public class ReflectionDemo {
    public static void main(String[] args) throws Exception {
        Class<?> clzss = Class.forName("com.java.reflect.ReflectionDemo");
        // target方法是訪問(wèn)權(quán)限是public，
        // 如果是private則需要通過(guò)clzss.getDeclaredMethod()獲取
        Method target = clzss.getMethod("target", int.class);
        target.invoke(null, 128);
    }

    public static void target(int i) {
      //輸出堆棧信息
        new Exception("#" + i).printStackTrace();
        System.out.println("invoke target method");
    }
}
----------------輸出---------------
$ javac com/java/reflect/ReflectionDemo.java 
$ java com/java/reflect/ReflectionDemo
java.lang.Exception: #128
        at com.java.reflect.ReflectionDemo.target(ReflectionDemo.java:18)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at com.java.reflect.ReflectionDemo.main(ReflectionDemo.java:14)
invoke target method

    源碼分析-調(diào)用鏈路時(shí)序圖

    反射調(diào)用鏈路時(shí)序圖.png

    源碼分析-調(diào)用鏈代碼

    step 1:調(diào)用代碼

    package com.java.reflect;
import java.lang.reflect.Method;
/**
 * 反射調(diào)用的例子
 * @author helenlee
 * @date 2018/9/4
 */
public class ReflectionDemo {
    public static void main(String[] args) throws Exception {
        Class<?> clzss = Class.forName("com.java.reflect.ReflectionDemo");
        // target方法是訪問(wèn)權(quán)限是public，
        // 如果是private則需要通過(guò)clzss.getDeclaredMethod()獲取
        Method target = clzss.getMethod("target", int.class);
        // step 2:Method執(zhí)行鏈路
        target.invoke(null, 128);
    }
    public static void target(int i) {
        new Exception("#" + i).printStackTrace();
        System.out.println("invoke target method");
    }
    public String targetString(int i,String name){
        new Exception("#" + i).printStackTrace();
        System.out.println("invoke target method");
        return name;
    }
}

    step 2:Method執(zhí)行鏈路

    package java.lang.reflect;
import sun.reflect.CallerSensitive;
import sun.reflect.MethodAccessor;
import sun.reflect.Reflection;
import sun.reflect.generics.repository.MethodRepository;
import sun.reflect.generics.factory.CoreReflectionFactory;
import sun.reflect.generics.factory.GenericsFactory;
import sun.reflect.generics.scope.MethodScope;
import sun.reflect.annotation.AnnotationType;
import sun.reflect.annotation.AnnotationParser;
import java.lang.annotation.Annotation;
import java.lang.annotation.AnnotationFormatError;
import java.nio.ByteBuffer;

public final class Method extends Executable {
    private Class<?>            clazz;
    private int                 modifiers;
    private volatile MethodAccessor methodAccessor;
    private Method              root;

    @CallerSensitive
    public Object invoke(Object obj, Object... args)
            throws IllegalAccessException, IllegalArgumentException,
            InvocationTargetException {
        // 1.權(quán)限檢查
        if (!override) {
            if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
                Class<?> caller = Reflection.getCallerClass();
                checkAccess(caller, clazz, obj, modifiers);
            }
        }
        // 2.獲取委派的對(duì)象
        MethodAccessor ma = methodAccessor;
        if (ma == null) {
            // step 3：獲取委派對(duì)象
            ma = acquireMethodAccessor();
        }
        // 3.委派或者動(dòng)態(tài)實(shí)現(xiàn)方式執(zhí)行調(diào)用，最終會(huì)調(diào)用到目標(biāo)方法
        // step 4：委派或者動(dòng)態(tài)實(shí)現(xiàn)方式執(zhí)行調(diào)用
        return ma.invoke(obj, args);
    }
}

    step 3：獲取委派對(duì)象

    private MethodAccessor acquireMethodAccessor() {
        MethodAccessor tmp = null;
        // 如果緩存中有則直接返回
        if (root != null) tmp = root.getMethodAccessor();
        if (tmp != null) {
            methodAccessor = tmp;
        } else {
            // 如果不存在則創(chuàng)建
            // step 3.1：創(chuàng)建MethodAccessor
            tmp = reflectionFactory.newMethodAccessor(this);
            setMethodAccessor(tmp);
        }
        return tmp;
    }

    step 3.1：創(chuàng)建MethodAccessor

    public MethodAccessor newMethodAccessor(Method method) {
        checkInitted();

        //如果不使用膨脹模式-Dsun.reflect.noInflation=true
        if (noInflation && !ReflectUtil.isVMAnonymousClass(method.getDeclaringClass())) {
            //通過(guò)ASM動(dòng)態(tài)生成
            return new MethodAccessorGenerator().
                generateMethod(method.getDeclaringClass(),
                               method.getName(),
                               method.getParameterTypes(),
                               method.getReturnType(),
                               method.getExceptionTypes(),
                               method.getModifiers());
        } else {
            // 委派模式，最終還是有NativeMethodAccessorImpl調(diào)用Native方法完成
            NativeMethodAccessorImpl acc =
                new NativeMethodAccessorImpl(method);
            DelegatingMethodAccessorImpl res =
                new DelegatingMethodAccessorImpl(acc);
            acc.setParent(res);
            return res;
        }
    }

    step 4：委派模式

    class DelegatingMethodAccessorImpl extends MethodAccessorImpl {
    private MethodAccessorImpl delegate;

    DelegatingMethodAccessorImpl(MethodAccessorImpl delegate) {
        setDelegate(delegate);
    }

    public Object invoke(Object obj, Object[] args)
        throws IllegalArgumentException, InvocationTargetException
    {
        // delegate是一個(gè)NativeMethodAccessorImpl對(duì)象
        return delegate.invoke(obj, args);
    }

    void setDelegate(MethodAccessorImpl delegate) {
        this.delegate = delegate;
    }
}
class NativeMethodAccessorImpl extends MethodAccessorImpl {
    private final Method method;
    private DelegatingMethodAccessorImpl parent;
    private int numInvocations;

    NativeMethodAccessorImpl(Method method) {
        this.method = method;
    }

    public Object invoke(Object obj, Object[] args)
        throws IllegalArgumentException, InvocationTargetException
    {
        // 如果執(zhí)行次數(shù)超過(guò)了ReflectionFactory.inflationThreshold()則使用動(dòng)態(tài)實(shí)現(xiàn)方式
        if (++numInvocations > ReflectionFactory.inflationThreshold()
                && !ReflectUtil.isVMAnonymousClass(method.getDeclaringClass())) {
            MethodAccessorImpl acc = (MethodAccessorImpl)
                new MethodAccessorGenerator().
                    generateMethod(method.getDeclaringClass(),
                                   method.getName(),
                                   method.getParameterTypes(),
                                   method.getReturnType(),
                                   method.getExceptionTypes(),
                                   method.getModifiers());
            parent.setDelegate(acc);
        }
        // 委派模式最終執(zhí)行的是native方法(整個(gè)過(guò)程Java->C->Java)
        return invoke0(method, obj, args);
    }

    void setParent(DelegatingMethodAccessorImpl parent) {
        this.parent = parent;
    }

    private static native Object invoke0(Method m, Object obj, Object[] args);
}

    step 4：動(dòng)態(tài)模式（靜態(tài)方法）

    package com.java.reflect;

import sun.reflect.MethodAccessorImpl;

import java.lang.reflect.InvocationTargetException;

/**
 * 動(dòng)態(tài)實(shí)現(xiàn)方式：
 * 由ASM字節(jié)碼生成被調(diào)用方法的MethodAccessor類(lèi)（相當(dāng)于加了中間層，由中間層直接調(diào)用目標(biāo)方法）
 *
 */
public class GeneratedMethodAccessor1 extends MethodAccessorImpl {
    public GeneratedMethodAccessor1() {
    }

    public Object invoke(Object var1, Object[] var2) throws InvocationTargetException {
        int var10000;
        try {
            //目標(biāo)方法只有一個(gè)參數(shù)
            if (var2.length != 1) {
                throw new IllegalArgumentException();
            }

            Object var3 = var2[0];
            if (var3 instanceof Byte) {
                var10000 = (Byte)var3;
            } else if (var3 instanceof Character) {
                var10000 = (Character)var3;
            } else if (var3 instanceof Short) {
                var10000 = (Short)var3;
            } else {
                if (!(var3 instanceof Integer)) {
                    throw new IllegalArgumentException();
                }

                var10000 = (Integer)var3;
            }
        } catch (NullPointerException | ClassCastException var5) {
            throw new IllegalArgumentException(var5.toString());
        }

        try {
            //目標(biāo)方法是靜態(tài)方法（如果是實(shí)例方法則需要傳入的實(shí)例對(duì)象發(fā)起調(diào)用）
            ReflectionDemo.target(var10000);
            return null;
        } catch (Throwable var4) {
            throw new InvocationTargetException(var4);
        }
    }
}

    step 4：動(dòng)態(tài)模式（實(shí)例方法）

    package sun.reflect;

import com.java.reflect.ReflectionDemo;
import java.lang.reflect.InvocationTargetException;

/**
 * 實(shí)例方法生成動(dòng)態(tài)MethodAccessor類(lèi)
 * 如：public String targetString(int i,String name)；
 */
public class GeneratedMethodAccessor1 extends MethodAccessorImpl {
    public GeneratedMethodAccessor1() {
    }

    public Object invoke(Object var1, Object[] var2) throws InvocationTargetException {
        // 傳入的實(shí)例對(duì)象不能為空
        if (var1 == null) {
            throw new NullPointerException();
        } else {
            ReflectionDemo var10000;
            int var10001;
            String var10002;
            try {
                // 強(qiáng)轉(zhuǎn)實(shí)例對(duì)象
                var10000 = (ReflectionDemo)var1;
                // 入?yún)€(gè)數(shù)校驗(yàn)
                if (var2.length != 2) {
                    throw new IllegalArgumentException();
                }
                // 入?yún)㈩?lèi)型轉(zhuǎn)換
                Object var3 = var2[0];
                if (var3 instanceof Byte) {
                    var10001 = (Byte)var3;
                } else if (var3 instanceof Character) {
                    var10001 = (Character)var3;
                } else if (var3 instanceof Short) {
                    var10001 = (Short)var3;
                } else {
                    if (!(var3 instanceof Integer)) {
                        throw new IllegalArgumentException();
                    }
                    var10001 = (Integer)var3;
                }
                var10002 = (String)var2[1];
            } catch (NullPointerException | ClassCastException var5) {
                throw new IllegalArgumentException(var5.toString());
            }

            try {
                // 實(shí)例對(duì)象調(diào)用目標(biāo)方法
                return var10000.targetString(var10001, var10002);
            } catch (Throwable var4) {
                throw new InvocationTargetException(var4);
            }
        }
    }
}
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