前言:
眼前我们对“shapejava”大体比较看重,大家都想要剖析一些“shapejava”的相关内容。那么小编也在网摘上网罗了一些有关“shapejava””的相关文章,希望各位老铁们能喜欢,看官们快快来了解一下吧!目录:
一.反射基础
二.反射的作用
三.反射机制执行的流程
一.反射基础
什么是反射?
反射使 Java 代码可以发现有关已加载类的字段,方法和构造函数的信息,并在安全性限制内使用反射对这些字段,方法和构造函数进行操作。
反射的概念是由Smith在1982年首次提出的,主要是指程序可以访问、检测和修改它本身状态或行为的一种能力。这一概念的提出很快引发了计算机科学领域关于应用反射性的研究。它首先被程序语言的设计领域所采用,并在Lisp和面向对象方面取得了成绩。其中LEAD/LEAD++ 、OpenC++ 、MetaXa和OpenJava等就是基于反射机制的语言。最近,反射机制也被应用到了视窗系统、操作系统和文件系统中。
Java中,反射是一种强大的工具。它使您能够创建灵活的代码,这些代码可以在运行时装配,无需在组件之间进行源代表链接。反射允许我们在编写与执行时,使我们的程序代码能够接入装载到JVM中的类的内部信息,而不是源代码中选定的类协作的代码。这使反射成为构建灵活的应用的主要工具。但需注意的是:如果使用不当,反射的成本很高。
简而言之,指在 Java 程序运行时
给定的一个类(Class)对象,通过反射获取这个类(Class)对象的所有成员结构。给定的一个具体的对象,能够动态地调用它的方法及对任意属性值进行获取和赋值。
这种动态获取类的内容,创建对象、以及动态调用对象的方法及操作属性的机制为反射。即使该对象的类型在编译期间是未知,该类的 .class 文件不存在,也可以通过反射直接创建对象。
优势
增加程序的灵活性,避免将固有的逻辑程序写死到代码里代码简洁,可读性强,可提高代码的复用率
劣势
相较直接调用,在量大的情景下反射性能下降存在一些内部暴露和安全隐患
为什么要有反射
有了反射,我们可以做以下事情:
在运行时检查一个对象在运行时,根据一个class构造一个对象在运行时,检查一个对象的属性和方法在运行时,调用一个对象的任意一个方法在运行时,改变对象的构造函数,属性,方法的可见性等等
反射是很多框架的共有的方法:
例如JUnit,就是使用反射去找出那些带有@Test注解的方法,然后就利用反射在单元测试中调用这些方法在web框架中,开发人员将他们定义实现的接口和类放到配置文件中,使用反射,他可以动态地在运行时自动初始化这些类和接口 例如,Spring中一般这样使用配置文件:
<bean id="someID" class="com.programcreek.Foo">
<property name="someField" value="someValue" /></bean>
当Spring读取到bean文件的时候,会调用Class.forName(String)方法"com.programcreek.Foo"来初始化这个类,然后在使用反射正确的get到所配置的属性的set方法,并把相应的值set进去。
Servlet web 也是使用这种反射技术:
<servlet>
<servlet-name>someServlet</servlet-name>
<servlet-class>com.programcreek.WhyReflectionServlet</servlet-class><servlet>
反射的原理(类加载)
关于类加载机制,大家可以参考我的这篇文章:
深入理解JVM虚拟机——类的加载机制
深入理解JVM虚拟机——JVM是如何实现反射的?
类加载机制流程
类的加载
反射的原理图解
二. 反射的作用
一个类的成员包括以下三种:域信息、构造器信息、方法信息。而反射则可以在运行时动态获取到这些信息,在使用反射时,我们常用的类有以下五种。
Class类对象的获取
1、获得Class:主要有三种方法:
(1)Object–>getClass
(2)任何数据类型(包括基本的数据类型)都有一个“静态”的class属性
(3)通过class类的静态方法:forName(String className)(最常用)
package fanshe;public class Fanshe { public static void main(String[] args) { //第一种方式获取Class对象 Student stu1 = new Student();//这一new 产生一个Student对象,一个Class对象。 Class stuClass = stu1.getClass();//获取Class对象 System.out.println(stuClass.getName()); //第二种方式获取Class对象 Class stuClass2 = Student.class; System.out.println(stuClass == stuClass2);//判断第一种方式获取的Class对象和第二种方式获取的是否是同一个 //第三种方式获取Class对象 try { Class stuClass3 = Class.forName("fanshe.Student");//注意此字符串必须是真实路径,就是带包名的类路径,包名.类名 System.out.println(stuClass3 == stuClass2);//判断三种方式是否获取的是同一个Class对象 } catch (ClassNotFoundException e) { e.printStackTrace(); } }}
注意,在运行期间,一个类,只有一个Class对象产生,所以打印结果都是true;
三种方式中,常用第三种,第一种对象都有了还要反射干什么,第二种需要导入类包,依赖太强,不导包就抛编译错误。一般都使用第三种,一个字符串可以传入也可以写在配置文件中等多种方法。
Class类的方法
getName、getCanonicalName与getSimpleName的区别:
getSimpleName:只获取类名getName:类的全限定名,jvm中Class的表示,可以用于动态加载Class对象,例如Class.forName。getCanonicalName:返回更容易理解的表示,主要用于输出(toString)或log打印,大多数情况下和getName一样,但是在内部类、数组等类型的表示形式就不同了。
Constructor类及其获取对象方法
Constructor提供了一个类的单个构造函数的信息和访问。Constructor允许在将实际参数与newInstance()与底层构造函数的形式参数进行匹配时进行扩展转换,但如果发生缩小转换,则抛出IllegalArgumentException 。
Constructor类的方法
获取Constructor对象是通过Class类中的方法获取的,Class类与Constructor相关的主要方法如下:
使用反射技术获取构造器对象并使用
@Testpublic void test2() throws NoSuchMethodException { Class<Student> sc = Student.class; // 1. 拿到所有的构造器 Constructor<?>[] constructors = sc.getDeclaredConstructors(); // 输出构造器的名称+参数个数 for (Constructor<?> constructor : constructors) { System.out.println(constructor.getName() + " 参数个数:" + constructor.getParameterCount() + "个"); } // 2. 拿到单个构造器 Constructor<Student> constructor = sc.getDeclaredConstructor(String.class, String.class); System.out.println(constructor.getName() + "参数个数:" + constructor.getParameterCount());}
使用反射技术获取构造器对象并使用获取到的内容创建出一个对象
反射得到构造器之后的作用仍是创建一个对象,如果说构造器是public,就可以直接new对象,如果说是构造器是私有的private,需要提前将构造器进行暴力反射,再进行构造对象。
反射是可以直接破换掉封装性的,私有的也是可以执行的。
Field类及其用法
Field 提供有关类或接口的单个字段的信息,以及对它的动态访问权限。反射的字段可能是一个类(静态)字段或实例字段。
Field类涉及的get方法
同样的道理,我们可以通过Class类的提供的方法来获取代表字段信息的Field对象,Class类与Field对象相关方法如下:
下面的代码演示了上述方法的使用过程
public class ReflectField { public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException { Class<?> clazz = Class.forName("reflect.Student"); //获取指定字段名称的Field类,注意字段修饰符必须为public而且存在该字段, // 否则抛NoSuchFieldException Field field = clazz.getField("age"); System.out.println("field:" + field); //获取所有修饰符为public的字段,包含父类字段,注意修饰符为public才会获取 Field fields[] = clazz.getFields(); for (Field f : fields) { System.out.println("f:" + f.getDeclaringClass()); } System.out.println("================getDeclaredFields===================="); //获取当前类所字段(包含private字段),注意不包含父类的字段 Field fields2[] = clazz.getDeclaredFields(); for (Field f : fields2) { System.out.println("f2:" + f.getDeclaringClass()); } //获取指定字段名称的Field类,可以是任意修饰符的自动,注意不包含父类的字段 Field field2 = clazz.getDeclaredField("desc"); System.out.println("field2:" + field2); }/** 输出结果: field:public int reflect.Person.age f:public java.lang.String reflect.Student.desc f:public int reflect.Person.age f:public java.lang.String reflect.Person.name ================getDeclaredFields==================== f2:public java.lang.String reflect.Student.desc f2:private int reflect.Student.score field2:public java.lang.String reflect.Student.desc */}class Person { public int age; public String name;//省略set和get方法} class Student extends Person { public String desc; private int score; //省略set和get方法 }
上述方法需要注意的是,如果我们不期望获取其父类的字段,则需使用Class类的getDeclaredField/getDeclaredFields方法来获取字段即可,倘若需要连带获取到父类的字段,那么请使用Class类的getField/getFields,但是也只能获取到public修饰的的字段,无法获取父类的私有字段。下面将通过Field类本身的方法对指定类属性赋值,代码演示如下:
//获取Class对象引用
Class<?> clazz = Class.forName("reflect.Student");
Student st= (Student) clazz.newInstance();
//获取父类public字段并赋值
Field ageField = clazz.getField("age");
ageField.set(st,18);
Field nameField = clazz.getField("name");
nameField.set(st,"Lily");
//只获取当前类的字段,不获取父类的字段
Field descField = clazz.getDeclaredField("desc");
descField.set(st,"I am student");Field scoreField = clazz.getDeclaredField("score");
//设置可访问,score是private的
scoreField.setAccessible(true);
scoreField.set(st,88);System.out.println(st.toString());
//输出结果:Student{age=18, name='Lily ,desc='I am student', score=88}
//获取字段值System.out.println(scoreField.get(st));// 88
其中的set(Object obj, Object value)方法是Field类本身的方法,用于设置字段的值,而get(Object obj)则是获取字段的值,当然关于Field类还有其他常用的方法如下:
上述方法可能是较为常用的,事实上在设置值的方法上,Field类还提供了专门针对基本数据类型的方法,如setInt()/getInt()、setBoolean()/getBoolean、setChar()/getChar()等等方法,这里就不全部列出了,需要时查API文档即可。需要特别注意的是被final关键字修饰的Field字段是安全的,在运行时可以接收任何修改,但最终其实际值是不会发生改变的。
Method类及其用法
Method 提供关于类或接口上单独某个方法(以及如何访问该方法)的信息,所反映的方法可能是类方法或实例方法(包括抽象方法)。
Method类的主要方法
下面是Class类获取Method对象相关的方法:
同样通过案例演示上述方法:
import java.lang.reflect.Method;public class ReflectMethod { public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException { Class clazz = Class.forName("reflect.Circle"); //根据参数获取public的Method,包含继承自父类的方法 Method method = clazz.getMethod("draw", int.class, String.class); System.out.println("method:" + method); //获取所有public的方法: Method[] methods = clazz.getMethods(); for (Method m : methods) { System.out.println("m::" + m); } System.out.println("========================================="); //获取当前类的方法包含private,该方法无法获取继承自父类的method Method method1 = clazz.getDeclaredMethod("drawCircle"); System.out.println("method1::" + method1); //获取当前类的所有方法包含private,该方法无法获取继承自父类的method Method[] methods1 = clazz.getDeclaredMethods(); for (Method m : methods1) { System.out.println("m1::" + m); } }}class Shape { public void draw() { System.out.println("draw"); } public void draw(int count, String name) { System.out.println("draw " + name + ",count=" + count); }}class Circle extends Shape { private void drawCircle() { System.out.println("drawCircle"); } public int getAllCount() { return 100; }}
输出结果:
method:public void reflect.Shape.draw(int,java.lang.String)
m::public int reflect.Circle.getAllCount()
m::public void reflect.Shape.draw()
m::public void reflect.Shape.draw(int,java.lang.String)
m::public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
m::public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
m::public final void java.lang.Object.wait() throws java.lang.InterruptedException
m::public boolean java.lang.Object.equals(java.lang.Object)
m::public java.lang.String java.lang.Object.toString()
m::public native int java.lang.Object.hashCode()
m::public final native java.lang.Class java.lang.Object.getClass()
m::public final native void java.lang.Object.notify()
m::public final native void java.lang.Object.notifyAll()
=========================================
method1::private void reflect.Circle.drawCircle()
m1::public int reflect.Circle.getAllCount()
m1::private void reflect.Circle.drawCircle()
在通过getMethods方法获取Method对象时,会把父类的方法也获取到,如上的输出结果,把Object类的方法都打印出来了。而getDeclaredMethod/getDeclaredMethods方法都只能获取当前类的方法。我们在使用时根据情况选择即可。下面将演示通过Method对象调用指定类的方法:
Class clazz = Class.forName("reflect.Circle");
//创建对象
Circle circle = (Circle) clazz.newInstance();
//获取指定参数的方法对象
MethodMethod method = clazz.getMethod("draw",int.class,String.class);
//通过Method对象的invoke(Object obj,Object... args)方法调用
method.invoke(circle,15,"圈圈");
//对私有无参方法的操作
Method method1 = clazz.getDeclaredMethod("drawCircle");
//修改私有方法的访问标识
method1.setAccessible(true);
method1.invoke(circle);
//对有返回值得方法操作
Method method2 =clazz.getDeclaredMethod("getAllCount");
Integer count = (Integer) method2.invoke(circle);
System.out.println("count:"+count);
输出结果
draw 圈圈,count=15
drawCircle
count:100
在上述代码中调用方法,使用了Method类的invoke(Object obj,Object... args)第一个参数代表调用的对象,第二个参数传递的调用方法的参数。这样就完成了类方法的动态调用。
三. 反射机制执行的流程
-- 测试代码
-- 执行流程图
反射获取类实例
首先调用了 java.lang.Class 的静态方法,获取类信息。
@CallerSensitivepublic static Class<?> forName(String className) throws ClassNotFoundException { // 先通过反射,获取调用进来的类信息,从而获取当前的 classLoader Class<?> caller = Reflection.getCallerClass(); // 调用native方法进行获取class信息 return forName0(className, true, ClassLoader.getClassLoader(caller), caller);}
forName()反射获取类信息,并没有将实现留给了java,而是交给了jvm去加载。
主要是先获取 ClassLoader, 然后调用 native 方法,获取信息,加载类则是回调 java.lang.ClassLoader.
最后,jvm又会回调 ClassLoader 进类加载。
// public Class<?> loadClass(String name) throws ClassNotFoundException { return loadClass(name, false); }// sun.misc.Launcher public Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException { int var3 = var1.lastIndexOf(46); if(var3 != -1) { SecurityManager var4 = System.getSecurityManager(); if(var4 != null) { var4.checkPackageAccess(var1.substring(0, var3)); } } if(this.ucp.knownToNotExist(var1)) { Class var5 = this.findLoadedClass(var1); if(var5 != null) { if(var2) { this.resolveClass(var5); } return var5; } else { throw new ClassNotFoundException(var1); } } else { return super.loadClass(var1, var2); } }// java.lang.ClassLoader protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {// 先获取锁 synchronized (getClassLoadingLock(name)) {// First, check if the class has already been loaded// 如果已经加载了的话,就不用再加载了 Class<?> c = findLoadedClass(name); if (c == null) { long t0 = System.nanoTime(); try {// 双亲委托加载 if (parent != null) { c = parent.loadClass(name, false); } else { c = findBootstrapClassOrNull(name); } } catch (ClassNotFoundException e) {// ClassNotFoundException thrown if class not found// from the non-null parent class loader }// 父类没有加载到时,再自己加载 if (c == null) {// If still not found, then invoke findClass in order// to find the class. long t1 = System.nanoTime(); c = findClass(name);// this is the defining class loader; record the stats sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0); sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1); sun.misc.PerfCounter.getFindClasses().increment(); } } if (resolve) { resolveClass(c); } return c; } } protected Object getClassLoadingLock(String className) { Object lock = this; if (parallelLockMap != null) {// 使用 ConcurrentHashMap来保存锁 Object newLock = new Object(); lock = parallelLockMap.putIfAbsent(className, newLock); if (lock == null) { lock = newLock; } } return lock; } protected final Class<?> findLoadedClass(String name) { if (!checkName(name)) return null; return findLoadedClass0(name); }
下面来看一下 newInstance() 的实现方式。
// 首先肯定是 Class.newInstance @CallerSensitive public T newInstance() throws InstantiationException, IllegalAccessException { if (System.getSecurityManager() != null) { checkMemberAccess(Member.PUBLIC, Reflection.getCallerClass(), false); }// NOTE: the following code may not be strictly correct under// the current Java memory model.// Constructor lookup// newInstance() 其实相当于调用类的无参构造函数,所以,首先要找到其无参构造器 if (cachedConstructor == null) { if (this == Class.class) {// 不允许调用 Class 的 newInstance() 方法 throw new IllegalAccessException( "Can not call newInstance() on the Class for java.lang.Class" ); } try {// 获取无参构造器 Class<?>[] empty = {}; final Constructor<T> c = getConstructor0(empty, Member.DECLARED);// Disable accessibility checks on the constructor// since we have to do the security check here anyway// (the stack depth is wrong for the Constructor's// security check to work) java.security.AccessController.doPrivileged( new java.security.PrivilegedAction<Void>() { public Void run() { c.setAccessible(true); return null; } }); cachedConstructor = c; } catch (NoSuchMethodException e) { throw (InstantiationException) new InstantiationException(getName()).initCause(e); } } Constructor<T> tmpConstructor = cachedConstructor; // Security check (same as in java.lang.reflect.Constructor) int modifiers = tmpConstructor.getModifiers(); if (!Reflection.quickCheckMemberAccess(this, modifiers)) { Class<?> caller = Reflection.getCallerClass(); if (newInstanceCallerCache != caller) { Reflection.ensureMemberAccess(caller, this, null, modifiers); newInstanceCallerCache = caller; } } // Run constructor try { // 调用无参构造器 return tmpConstructor.newInstance((Object[]) null); } catch (InvocationTargetException e) { Unsafe.getUnsafe().throwException(e.getTargetException()); // Not reached return null; } }
newInstance() 主要做了三件事:
权限检测,如果不通过直接抛出异常;查找无参构造器,并将其缓存起来;调用具体方法的无参构造方法,生成实例并返回;
下面是获取构造器的过程:
private Constructor<T> getConstructor0(Class<?>[] parameterTypes, int which) throws NoSuchMethodException {// 获取所有构造器 Constructor<T>[] constructors = privateGetDeclaredConstructors((which == Member.PUBLIC)); for (Constructor<T> constructor : constructors) { if (arrayContentsEq(parameterTypes, constructor.getParameterTypes())) { return getReflectionFactory().copyConstructor(constructor); } } throw new NoSuchMethodException(getName() + ".<init>" + argumentTypesToString(parameterTypes)); }
getConstructor0() 为获取匹配的构造方器;分三步:
先获取所有的constructors, 然后通过进行参数类型比较;找到匹配后,通过 ReflectionFactory copy一份constructor返回;否则抛出 NoSuchMethodException;
// 获取当前类所有的构造方法,通过jvm或者缓存// Returns an array of "root" constructors. These Constructor// objects must NOT be propagated to the outside world, but must// instead be copied via ReflectionFactory.copyConstructor. private Constructor<T>[] privateGetDeclaredConstructors(boolean publicOnly) { checkInitted(); Constructor<T>[] res;// 调用 reflectionData(), 获取保存的信息,使用软引用保存,从而使内存不够可以回收 ReflectionData<T> rd = reflectionData(); if (rd != null) { res = publicOnly ? rd.publicConstructors : rd.declaredConstructors;// 存在缓存,则直接返回 if (res != null) return res; }// No cached value available; request value from VM if (isInterface()) { @SuppressWarnings("unchecked") Constructor<T>[] temporaryRes = (Constructor<T>[]) new Constructor<?>[0]; res = temporaryRes; } else {// 使用native方法从jvm获取构造器 res = getDeclaredConstructors0(publicOnly); } if (rd != null) {// 最后,将从jvm中读取的内容,存入缓存 if (publicOnly) { rd.publicConstructors = res; } else { rd.declaredConstructors = res; } } return res; }// Lazily create and cache ReflectionData private ReflectionData<T> reflectionData() { SoftReference<ReflectionData<T>> reflectionData = this.reflectionData; int classRedefinedCount = this.classRedefinedCount; ReflectionData<T> rd; if (useCaches && reflectionData != null && (rd = reflectionData.get()) != null && rd.redefinedCount == classRedefinedCount) { return rd; }// else no SoftReference or cleared SoftReference or stale ReflectionData// -> create and replace new instance return newReflectionData(reflectionData, classRedefinedCount); }// 新创建缓存,保存反射信息 private ReflectionData<T> newReflectionData(SoftReference<ReflectionData<T>> oldReflectionData, int classRedefinedCount) { if (!useCaches) return null;// 使用cas保证更新的线程安全性,所以反射是保证线程安全的 while (true) { ReflectionData<T> rd = new ReflectionData<>(classRedefinedCount);// try to CAS it... if (Atomic.casReflectionData(this, oldReflectionData, new SoftReference<>(rd))) { return rd; }// 先使用CAS更新,如果更新成功,则立即返回,否则测查当前已被其他线程更新的情况,如果和自己想要更新的状态一致,则也算是成功了 oldReflectionData = this.reflectionData; classRedefinedCount = this.classRedefinedCount; if (oldReflectionData != null && (rd = oldReflectionData.get()) != null && rd.redefinedCount == classRedefinedCount) { return rd; } } }
如上,privateGetDeclaredConstructors(), 获取所有的构造器主要步骤;
先尝试从缓存中获取;如果缓存没有,则从jvm中重新获取,并存入缓存,缓存使用软引用进行保存,保证内存可用;
另外,使用 relactionData() 进行缓存保存;ReflectionData 的数据结构如下。
// reflection data that might get invalidated when JVM TI RedefineClasses() is called private static class ReflectionData<T> { volatile Field[] declaredFields; volatile Field[] publicFields; volatile Method[] declaredMethods; volatile Method[] publicMethods; volatile Constructor<T>[] declaredConstructors; volatile Constructor<T>[] publicConstructors;// Intermediate results for getFields and getMethods volatile Field[] declaredPublicFields; volatile Method[] declaredPublicMethods; volatile Class<?>[] interfaces;// Value of classRedefinedCount when we created this ReflectionData instance final int redefinedCount; ReflectionData(int redefinedCount) { this.redefinedCount = redefinedCount; } }
其中,还有一个点,就是如何比较构造是否是要查找构造器,其实就是比较类型完成相等就完了,有一个不相等则返回false。
private static boolean arrayContentsEq(Object[] a1, Object[] a2) { if (a1 == null) { return a2 == null || a2.length == 0; } if (a2 == null) { return a1.length == 0; } if (a1.length != a2.length) { return false; } for (int i = 0; i < a1.length; i++) { if (a1[i] != a2[i]) { return false; } } return true; }// sun.reflect.ReflectionFactory /** * Makes a copy of the passed constructor. The returned * <p> * constructor is a "child" of the passed one; see the comments * <p> * in Constructor.java for details. */ public <T> Constructor<T> copyConstructor(Constructor<T> arg) { return langReflectAccess().copyConstructor(arg); }// java.lang.reflect.Constructor, copy 其实就是新new一个 Constructor 出来 Constructor<T> copy() {// This routine enables sharing of ConstructorAccessor objects// among Constructor objects which refer to the same underlying// method in the VM. (All of this contortion is only necessary// because of the "accessibility" bit in AccessibleObject,// which implicitly requires that new java.lang.reflect// objects be fabricated for each reflective call on Class// objects.) if (this.root != null) throw new IllegalArgumentException("Can not copy a non-root Constructor"); Constructor<T> res = new Constructor<>(clazz, parameterTypes, exceptionTypes, modifiers, slot, signature, annotations, parameterAnnotations);// root 指向当前 constructor res.root = this;// Might as well eagerly propagate this if already present res.constructorAccessor = constructorAccessor; return res; }
通过上面,获取到 Constructor 了。
接下来就只需调用其相应构造器的 newInstance(),即返回实例了。
// return tmpConstructor.newInstance((Object[])null);// java.lang.reflect.Constructor @CallerSensitive public T newInstance(Object... initargs) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException { if (!override) { if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) { Class<?> caller = Reflection.getCallerClass(); checkAccess(caller, clazz, null, modifiers); } } if ((clazz.getModifiers() & Modifier.ENUM) != 0) throw new IllegalArgumentException("Cannot reflectively create enum objects"); ConstructorAccessor ca = constructorAccessor; // read volatile if (ca == null) { ca = acquireConstructorAccessor(); } @SuppressWarnings("unchecked") T inst = (T) ca.newInstance(initargs); return inst; }// sun.reflect.DelegatingConstructorAccessorImpl public Object newInstance(Object[] args) throws InstantiationException, IllegalArgumentException, InvocationTargetException { return delegate.newInstance(args); }// sun.reflect.NativeConstructorAccessorImpl public Object newInstance(Object[] args) throws InstantiationException, IllegalArgumentException, InvocationTargetException {// We can't inflate a constructor belonging to a vm-anonymous class// because that kind of class can't be referred to by name, hence can't// be found from the generated bytecode. if (++numInvocations > ReflectionFactory.inflationThreshold() && !ReflectUtil.isVMAnonymousClass(c.getDeclaringClass())) { ConstructorAccessorImpl acc = (ConstructorAccessorImpl) new MethodAccessorGenerator(). generateConstructor(c.getDeclaringClass(), c.getParameterTypes(), c.getExceptionTypes(), c.getModifiers()); parent.setDelegate(acc); }// 调用native方法,进行调用 constructor return newInstance0(c, args); }
返回构造器的实例后,可以根据外部进行进行类型转换,从而使用接口或方法进行调用实例功能了。
反射获取方法
第一步,先获取 Method;
// java.lang.Class@CallerSensitivepublic Method getDeclaredMethod(String name, Class<?>... parameterTypes) throws NoSuchMethodException, SecurityException { checkMemberAccess(Member.DECLARED, Reflection.getCallerClass(), true); Method method = searchMethods(privateGetDeclaredMethods(false), name, parameterTypes); if (method == null) { throw new NoSuchMethodException(getName() + "." + name + argumentTypesToString(parameterTypes)); } return method;}
忽略第一个检查权限,剩下就只有两个动作了。
获取所有方法列表;根据方法名称和方法列表,选出符合要求的方法;如果没有找到相应方法,抛出异常,否则返回对应方法;
所以,先看一下怎样获取类声明的所有方法?
// Returns an array of "root" methods. These Method objects must NOT// be propagated to the outside world, but must instead be copied// via ReflectionFactory.copyMethod. private Method[] privateGetDeclaredMethods(boolean publicOnly) { checkInitted(); Method[] res; ReflectionData<T> rd = reflectionData(); if (rd != null) { res = publicOnly ? rd.declaredPublicMethods : rd.declaredMethods; if (res != null) return res; }// No cached value available; request value from VM res = Reflection.filterMethods(this, getDeclaredMethods0(publicOnly)); if (rd != null) { if (publicOnly) { rd.declaredPublicMethods = res; } else { rd.declaredMethods = res; } } return res; }
很相似,和获取所有构造器的方法很相似,都是先从缓存中获取方法,如果没有,则从jvm中获取。
不同的是,方法列表需要进行过滤 Reflection.filterMethods;当然后面看来,这个方法我们一般不会派上用场。
// sun.misc.Reflection public static Method[] filterMethods(Class<?> containingClass, Method[] methods) { if (methodFilterMap == null) {// Bootstrapping return methods; } return (Method[]) filter(methods, methodFilterMap.get(containingClass)); }// 可以过滤指定的方法,一般为空,如果要指定过滤,可以调用 registerMethodsToFilter(), 或者... private static Member[] filter(Member[] members, String[] filteredNames) { if ((filteredNames == null) || (members.length == 0)) { return members; } int numNewMembers = 0; for (Member member : members) { boolean shouldSkip = false; for (String filteredName : filteredNames) { if (member.getName() == filteredName) { shouldSkip = true; break; } } if (!shouldSkip) { ++numNewMembers; } } Member[] newMembers = (Member[]) Array.newInstance(members[0].getClass(), numNewMembers); int destIdx = 0; for (Member member : members) { boolean shouldSkip = false; for (String filteredName : filteredNames) { if (member.getName() == filteredName) { shouldSkip = true; break; } } if (!shouldSkip) { newMembers[destIdx++] = member; } } return newMembers; }
第二步,根据方法名和参数类型过滤指定方法返回:
private static Method searchMethods(Method[] methods, String name, Class<?>[] parameterTypes) { Method res = null;// 使用常量池,避免重复创建String String internedName = name.intern(); for (int i = 0; i < methods.length; i++) { Method m = methods[i]; if (m.getName() == internedName && arrayContentsEq(parameterTypes, m.getParameterTypes()) && (res == null || res.getReturnType().isAssignableFrom(m.getReturnType()))) res = m; } return (res == null ? res : getReflectionFactory().copyMethod(res)); }
大概意思看得明白,就是匹配到方法名,然后参数类型匹配,才可以。
但是可以看到,匹配到一个方法,并没有退出for循环,而是继续进行匹配。这里是匹配最精确的子类进行返回(最优匹配)最后,还是通过 ReflectionFactory, copy 方法后返回。
调用 method.invoke() 方法
@CallerSensitivepublic Object invoke(Object obj, Object... args) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException{ if (!override) { if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) { Class<?> caller = Reflection.getCallerClass(); checkAccess(caller, clazz, obj, modifiers); } } MethodAccessor ma = methodAccessor; // read volatile if (ma == null) { ma = acquireMethodAccessor(); } return ma.invoke(obj, args);}
invoke时,是通过 MethodAccessor 进行调用的,而 MethodAccessor 是个接口,在第一次时调用 acquireMethodAccessor() 进行新创建。
// probably make the implementation more scalable. private MethodAccessor acquireMethodAccessor() {// First check to see if one has been created yet, and take it// if so MethodAccessor tmp = null; if (root != null) tmp = root.getMethodAccessor(); if (tmp != null) {// 存在缓存时,存入 methodAccessor,否则调用 ReflectionFactory 创建新的 MethodAccessor methodAccessor = tmp; } else {// Otherwise fabricate one and propagate it up to the root tmp = reflectionFactory.newMethodAccessor(this); setMethodAccessor(tmp); } return tmp; }// sun.reflect.ReflectionFactory public MethodAccessor newMethodAccessor(Method method) { checkInitted(); if (noInflation && !ReflectUtil.isVMAnonymousClass(method.getDeclaringClass())) { return new MethodAccessorGenerator(). generateMethod(method.getDeclaringClass(), method.getName(), method.getParameterTypes(), method.getReturnType(), method.getExceptionTypes(), method.getModifiers()); } else { NativeMethodAccessorImpl acc = new NativeMethodAccessorImpl(method); DelegatingMethodAccessorImpl res = new DelegatingMethodAccessorImpl(acc); acc.setParent(res); return res; } }
两个Accessor详情:
// NativeMethodAccessorImpl / DelegatingMethodAccessorImplclass 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 {// We can't inflate methods belonging to vm-anonymous classes because// that kind of class can't be referred to by name, hence can't be// found from the generated bytecode. 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); } return invoke0(method, obj, args); } void setParent(DelegatingMethodAccessorImpl parent) { this.parent = parent; } private static native Object invoke0(Method m, Object obj, Object[] args);}class DelegatingMethodAccessorImpl extends MethodAccessorImpl { private MethodAccessorImpl delegate; DelegatingMethodAccessorImpl(MethodAccessorImpl delegate) { setDelegate(delegate); } public Object invoke(Object obj, Object[] args) throws IllegalArgumentException, InvocationTargetException { return delegate.invoke(obj, args); } void setDelegate(MethodAccessorImpl delegate) { this.delegate = delegate; }
进行 ma.invoke(obj, args); 调用时,调用 DelegatingMethodAccessorImpl.invoke();
最后被委托到 NativeMethodAccessorImpl.invoke(), 即:
public Object invoke(Object obj, Object[] args) throws IllegalArgumentException, InvocationTargetException {// We can't inflate methods belonging to vm-anonymous classes because// that kind of class can't be referred to by name, hence can't be// found from the generated bytecode. 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); }
// invoke0 是个 native 方法,由jvm进行调用业务方法。从而完成反射调用功能。
return invoke0(method, obj, args);
}
其中, generateMethod() 是生成具体类的方法:
/** * This routine is not thread-safe */public MethodAccessor generateMethod(Class<?> declaringClass, String name, Class<?>[] parameterTypes, Class<?> returnType, Class<?>[] checkedExceptions, int modifiers) { return (MethodAccessor) generate(declaringClass, name, parameterTypes, returnType, checkedExceptions, modifiers, false, false, null);}
generate() 戳详情。
/** * This routine is not thread-safe */private MagicAccessorImpl generate(final Class<?> declaringClass, String name, Class<?>[]parameterTypes, Class<?> returnType, Class<?>[]checkedExceptions, int modifiers, boolean isConstructor, boolean forSerialization, Class<?> serializationTargetClass) { ByteVector vec=ByteVectorFactory.create(); asm=new ClassFileAssembler(vec); this.declaringClass=declaringClass; this.parameterTypes=parameterTypes; this.returnType=returnType; this.modifiers=modifiers; this.isConstructor=isConstructor; this.forSerialization=forSerialization; asm.emitMagicAndVersion();// Constant pool entries:// ( * = Boxing information: optional)// (+ = Shared entries provided by AccessorGenerator)// (^ = Only present if generating SerializationConstructorAccessor)// [UTF-8] [This class's name]// [CONSTANT_Class_info] for above// [UTF-8] "sun/reflect/{MethodAccessorImpl,ConstructorAccessorImpl,SerializationConstructorAccessorImpl}"// [CONSTANT_Class_info] for above// [UTF-8] [Target class's name]// [CONSTANT_Class_info] for above// ^ [UTF-8] [Serialization: Class's name in which to invoke constructor]// ^ [CONSTANT_Class_info] for above// [UTF-8] target method or constructor name// [UTF-8] target method or constructor signature// [CONSTANT_NameAndType_info] for above// [CONSTANT_Methodref_info or CONSTANT_InterfaceMethodref_info] for target method// [UTF-8] "invoke" or "newInstance"// [UTF-8] invoke or newInstance descriptor// [UTF-8] descriptor for type of non-primitive parameter 1// [CONSTANT_Class_info] for type of non-primitive parameter 1// ...// [UTF-8] descriptor for type of non-primitive parameter n// [CONSTANT_Class_info] for type of non-primitive parameter n// + [UTF-8] "java/lang/Exception"// + [CONSTANT_Class_info] for above// + [UTF-8] "java/lang/ClassCastException"// + [CONSTANT_Class_info] for above// + [UTF-8] "java/lang/NullPointerException"// + [CONSTANT_Class_info] for above// + [UTF-8] "java/lang/IllegalArgumentException"// + [CONSTANT_Class_info] for above// + [UTF-8] "java/lang/InvocationTargetException"// + [CONSTANT_Class_info] for above// + [UTF-8] "<init>"// + [UTF-8] "()V"// + [CONSTANT_NameAndType_info] for above// + [CONSTANT_Methodref_info] for NullPointerException's constructor// + [CONSTANT_Methodref_info] for IllegalArgumentException's constructor// + [UTF-8] "(Ljava/lang/String;)V"// + [CONSTANT_NameAndType_info] for "<init>(Ljava/lang/String;)V"// + [CONSTANT_Methodref_info] for IllegalArgumentException's constructor taking a String// + [UTF-8] "(Ljava/lang/Throwable;)V"// + [CONSTANT_NameAndType_info] for "<init>(Ljava/lang/Throwable;)V"// + [CONSTANT_Methodref_info] for InvocationTargetException's constructor// + [CONSTANT_Methodref_info] for "super()"// + [UTF-8] "java/lang/Object"// + [CONSTANT_Class_info] for above// + [UTF-8] "toString"// + [UTF-8] "()Ljava/lang/String;"// + [CONSTANT_NameAndType_info] for "toString()Ljava/lang/String;"// + [CONSTANT_Methodref_info] for Object's toString method// + [UTF-8] "Code"// + [UTF-8] "Exceptions"// * [UTF-8] "java/lang/Boolean"// * [CONSTANT_Class_info] for above// * [UTF-8] "(Z)V"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "booleanValue"// * [UTF-8] "()Z"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "java/lang/Byte"// * [CONSTANT_Class_info] for above// * [UTF-8] "(B)V"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "byteValue"// * [UTF-8] "()B"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "java/lang/Character"// * [CONSTANT_Class_info] for above// * [UTF-8] "(C)V"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "charValue"// * [UTF-8] "()C"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "java/lang/Double"// * [CONSTANT_Class_info] for above// * [UTF-8] "(D)V"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "doubleValue"// * [UTF-8] "()D"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "java/lang/Float"// * [CONSTANT_Class_info] for above// * [UTF-8] "(F)V"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "floatValue"// * [UTF-8] "()F"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "java/lang/Integer"// * [CONSTANT_Class_info] for above// * [UTF-8] "(I)V"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "intValue"// * [UTF-8] "()I"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "java/lang/Long"// * [CONSTANT_Class_info] for above// * [UTF-8] "(J)V"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "longValue"// * [UTF-8] "()J"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "java/lang/Short"// * [CONSTANT_Class_info] for above// * [UTF-8] "(S)V"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above// * [UTF-8] "shortValue"// * [UTF-8] "()S"// * [CONSTANT_NameAndType_info] for above// * [CONSTANT_Methodref_info] for above short numCPEntries=NUM_BASE_CPOOL_ENTRIES+NUM_COMMON_CPOOL_ENTRIES; boolean usesPrimitives=usesPrimitiveTypes(); if(usesPrimitives){ numCPEntries+=NUM_BOXING_CPOOL_ENTRIES; } if(forSerialization){ numCPEntries+=NUM_SERIALIZATION_CPOOL_ENTRIES; }// Add in variable-length number of entries to be able to describe// non-primitive parameter types and checked exceptions. numCPEntries+=(short)(2*numNonPrimitiveParameterTypes()); asm.emitShort(add(numCPEntries,S1));final String generatedName=generateName(isConstructor,forSerialization); asm.emitConstantPoolUTF8(generatedName); asm.emitConstantPoolClass(asm.cpi()); thisClass=asm.cpi(); if(isConstructor){ if(forSerialization){ asm.emitConstantPoolUTF8 ("sun/reflect/SerializationConstructorAccessorImpl"); }else{ asm.emitConstantPoolUTF8("sun/reflect/ConstructorAccessorImpl"); } }else{ asm.emitConstantPoolUTF8("sun/reflect/MethodAccessorImpl"); } asm.emitConstantPoolClass(asm.cpi()); superClass=asm.cpi(); asm.emitConstantPoolUTF8(getClassName(declaringClass,false)); asm.emitConstantPoolClass(asm.cpi()); targetClass=asm.cpi(); short serializationTargetClassIdx=(short)0; if(forSerialization){ asm.emitConstantPoolUTF8(getClassName(serializationTargetClass,false)); asm.emitConstantPoolClass(asm.cpi()); serializationTargetClassIdx=asm.cpi(); } asm.emitConstantPoolUTF8(name); asm.emitConstantPoolUTF8(buildInternalSignature()); asm.emitConstantPoolNameAndType(sub(asm.cpi(),S1),asm.cpi()); if(isInterface()){ asm.emitConstantPoolInterfaceMethodref(targetClass,asm.cpi()); }else{ if(forSerialization){ asm.emitConstantPoolMethodref(serializationTargetClassIdx,asm.cpi()); }else{ asm.emitConstantPoolMethodref(targetClass,asm.cpi()); } } targetMethodRef=asm.cpi(); if(isConstructor){ asm.emitConstantPoolUTF8("newInstance"); }else{ asm.emitConstantPoolUTF8("invoke"); } invokeIdx=asm.cpi(); if(isConstructor){ asm.emitConstantPoolUTF8("([Ljava/lang/Object;)Ljava/lang/Object;"); }else{ asm.emitConstantPoolUTF8 ("(Ljava/lang/Object;[Ljava/lang/Object;)Ljava/lang/Object;"); } invokeDescriptorIdx=asm.cpi();// Output class information for non-primitive parameter types nonPrimitiveParametersBaseIdx=add(asm.cpi(),S2); for(int i=0;i<parameterTypes.length;i++){ Class<?> c=parameterTypes[i]; if(!isPrimitive(c)){ asm.emitConstantPoolUTF8(getClassName(c,false)); asm.emitConstantPoolClass(asm.cpi()); } }// Entries common to FieldAccessor, MethodAccessor and ConstructorAccessor emitCommonConstantPoolEntries();// Boxing entries if(usesPrimitives){ emitBoxingContantPoolEntries(); } if(asm.cpi()!=numCPEntries){ throw new InternalError("Adjust this code (cpi = "+asm.cpi()+ ", numCPEntries = "+numCPEntries+")"); }// Access flags asm.emitShort(ACC_PUBLIC);// This class asm.emitShort(thisClass);// Superclass asm.emitShort(superClass);// Interfaces count and interfaces asm.emitShort(S0);// Fields count and fields asm.emitShort(S0);// Methods count and methods asm.emitShort(NUM_METHODS); emitConstructor(); emitInvoke();// Additional attributes (none) asm.emitShort(S0);// Load class vec.trim();final byte[]bytes=vec.getData();// Note: the class loader is the only thing that really matters// here -- it's important to get the generated code into the// same namespace as the target class. Since the generated code// is privileged anyway, the protection domain probably doesn't// matter. return AccessController.doPrivileged( new PrivilegedAction<MagicAccessorImpl>(){public MagicAccessorImpl run(){ try{ return(MagicAccessorImpl) ClassDefiner.defineClass (generatedName, bytes, 0, bytes.length, declaringClass.getClassLoader()).newInstance(); }catch(InstantiationException|IllegalAccessException e){ throw new InternalError(e); } } }); }
咱们主要看这一句:ClassDefiner.defineClass(xx, declaringClass.getClassLoader()).newInstance();
在ClassDefiner.defineClass方法实现中,每被调用一次都会生成一个DelegatingClassLoader类加载器对象 ,这里每次都生成新的类加载器,是为了性能考虑,在某些情况下可以卸载这些生成的类,因为类的卸载是只有在类加载器可以被回收的情况下才会被回收的,如果用了原来的类加载器,那可能导致这些新创建的类一直无法被卸载。
而反射生成的类,有时候可能用了就可以卸载了,所以使用其独立的类加载器,从而使得更容易控制反射类的生命周期。
反射调用流程小结
最后,用几句话总结反射的实现原理:
反射类及反射方法的获取,都是通过从列表中搜寻查找匹配的方法,所以查找性能会随类的大小方法多少而变化;
每个类都会有一个与之对应的Class实例,从而每个类都可以获取method反射方法,并作用到其他实例身上;
反射也是考虑了线程安全的,放心使用;
反射使用软引用relectionData缓存class信息,避免每次重新从jvm获取带来的开销;
反射调用多次生成新代理Accessor, 而通过字节码生存的则考虑了卸载功能,所以会使用独立的类加载器;
当找到需要的方法,都会copy一份出来,而不是使用原来的实例,从而保证数据隔离;
调度反射方法,最终是由jvm执行invoke0()执行
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