org.springframework.test.util.ReflectionTestUtils.invokeMethod方法的使用

2022-12-07 01:55| 来源: 网络整理| 查看: 265

序言

为什么要用spring框架的ReflectionTestUtils工具类的invokeMethod方法？

当我们想要调用一个实例对象的私有方法时，我们可以利用反射机制去调用该私有方法。

Demo

含有私有方法的类，

public final class DemoClass {private static class Holder {private static final DemoClass instance = new DemoClass();}private String key;private void privateMethod(String input) {System.out.println("this is my private method! input parameter is :" + input);}public static final DemoClass getInstance() {return Holder.instance;} }

测试类，

import org.junit.Before; import org.junit.Test; import org.springframework.test.util.ReflectionTestUtils;import java.lang.reflect.Field;public class ReflectionTestUtilsDemo {@Beforepublic void beforeClassInit() {try {// 支持获取private属性Field declaredField = DemoClass.getInstance().getClass().getDeclaredField("key");System.out.println("getDeclaredField method can get private field, field is :" + declaredField.getName());// 仅可以获取public属性Field field = DemoClass.getInstance().getClass().getField("key");System.out.println("getField method can get private field, field is :" + field.getName());} catch (NoSuchFieldException e) {System.out.println("no such field exception");}}@Testpublic void test() {// 通过反射调用私有的非静态方法，第一个参数必须是对象实例ReflectionTestUtils.invokeMethod(DemoClass.getInstance(), "privateMethod", "ok");System.out.println("this is my test");} }

实际效果如下，

注意Class类的getDeclaredField才可以获取私有成员变量，getField方法只能获取公有成员变量。

代码解析

直接看org.springframework.test.util.ReflectionTestUtils类下面的invokeMethod方法，如下，

首先对目标对象断言不为空，只有对象不为null才会继续执行invokeMethod方法。

@Nullablepublic static T invokeMethod(Object target, String name, Object... args) {Assert.notNull(target, "Target object must not be null");return invokeMethod(target, (Class)null, name, args);}

invokeMethod的重载方法步骤如下，

1）判断目标对象或对象类不为null，且传进来的方法命不为空；2）创建MethodInvoker实例对象，并把目标对象或目标类，目标方法，参数等信息set到MethodInvoker对象中；3）调用MethodInvoker的prepare方法，这样后续可以多次直接调用反射获取的方法；4）调用MethodInvoker的invoke方法，执行方法。 @Nullablepublic static T invokeMethod(@Nullable Object targetObject, @Nullable Class targetClass, String name, Object... args) {Assert.isTrue(targetObject != null || targetClass != null, "Either 'targetObject' or 'targetClass' for the method must be specified");Assert.hasText(name, "Method name must not be empty");try {MethodInvoker methodInvoker = new MethodInvoker();methodInvoker.setTargetObject(targetObject);if (targetClass != null) {methodInvoker.setTargetClass(targetClass);}methodInvoker.setTargetMethod(name);methodInvoker.setArguments(args);methodInvoker.prepare();if (logger.isDebugEnabled()) {logger.debug(String.format("Invoking method '%s' on %s or %s with arguments %s", name, safeToString(targetObject), safeToString(targetClass), ObjectUtils.nullSafeToString(args)));}return methodInvoker.invoke();} catch (Exception var5) {ReflectionUtils.handleReflectionException(var5);throw new IllegalStateException("Should never get here");}}

下面看看prepare方法做了什么，

1）判断MethodInvoker对象传入的静态方法名是否不为空；2）拿到MethodInvoker对象传入的目标对象和目标方法，并断言不为空；3）获取参数集合和参数的类型；4）通过目标类的getMethod方法找父类方法或接口方法，找不到则通过MethodInvoker对象的findMatchingMethod方法，再找不到则抛异常； public void prepare() throws ClassNotFoundException, NoSuchMethodException {if (this.staticMethod != null) {int lastDotIndex = this.staticMethod.lastIndexOf('.');if (lastDotIndex == -1 || lastDotIndex == this.staticMethod.length()) {throw new IllegalArgumentException("staticMethod must be a fully qualified class plus method name: " +"e.g. 'example.MyExampleClass.myExampleMethod'");}String className = this.staticMethod.substring(0, lastDotIndex);String methodName = this.staticMethod.substring(lastDotIndex + 1);this.targetClass = resolveClassName(className);this.targetMethod = methodName;}Class targetClass = getTargetClass();String targetMethod = getTargetMethod();Assert.notNull(targetClass, "Either 'targetClass' or 'targetObject' is required");Assert.notNull(targetMethod, "Property 'targetMethod' is required");Object[] arguments = getArguments();Class[] argTypes = new Class[arguments.length];for (int i = 0; i < arguments.length; ++i) {argTypes[i] = (arguments[i] != null ? arguments[i].getClass() : Object.class);}// Try to get the exact method first.try {this.methodObject = targetClass.getMethod(targetMethod, argTypes);}catch (NoSuchMethodException ex) {// Just rethrow exception if we can't get any match.this.methodObject = findMatchingMethod();if (this.methodObject == null) {throw ex;}}}

继续往下挖，看看targetClass.getMethod怎么获取方法对象的，

1）检查是否允许客户端访问成员；2）从getMethod0方法中去找父类方法或接口方法；3）找不到则抛出NoSuchMethodException异常，找到则返回方法； @CallerSensitivepublic Method getMethod(String name, Class... parameterTypes)throws NoSuchMethodException, SecurityException {checkMemberAccess(Member.PUBLIC, Reflection.getCallerClass(), true);Method method = getMethod0(name, parameterTypes, true);if (method == null) {throw new NoSuchMethodException(getName() + "." + name + argumentTypesToString(parameterTypes));}return method;}

MethodInvoker对象的findMatchingMethod方法，

1）获取目标方法、参数、参数长度；2）获取目标类，断言不为null，并通过getAllDeclaredMethods方法获取目标类的所有方法；3）遍历目标类中的所有方法，找到匹配的方法； @Nullableprotected Method findMatchingMethod() {String targetMethod = getTargetMethod();Object[] arguments = getArguments();int argCount = arguments.length;Class targetClass = getTargetClass();Assert.state(targetClass != null, "No target class set");Method[] candidates = ReflectionUtils.getAllDeclaredMethods(targetClass);int minTypeDiffWeight = Integer.MAX_VALUE;Method matchingMethod = null;for (Method candidate : candidates) {if (candidate.getName().equals(targetMethod)) {if (candidate.getParameterCount() == argCount) {Class[] paramTypes = candidate.getParameterTypes();int typeDiffWeight = getTypeDifferenceWeight(paramTypes, arguments);if (typeDiffWeight < minTypeDiffWeight) {minTypeDiffWeight = typeDiffWeight;matchingMethod = candidate;}}}}return matchingMethod;}

再看方法的调用是如何实现的，找到MethodInvoker对象的invoke方法，

1）获取目标对象和提前准备好的方法；

2）如果目标对象为null或者目标方法是静态方法则抛出IllegalArgumentException异常；

3）使给定的非静态方法可访问setAccessible=true；

4）调用目标方法Method的invoke方法；

@Nullablepublic Object invoke() throws InvocationTargetException, IllegalAccessException {// In the static case, target will simply be {@code null}.Object targetObject = getTargetObject();Method preparedMethod = getPreparedMethod();if (targetObject == null && !Modifier.isStatic(preparedMethod.getModifiers())) {throw new IllegalArgumentException("Target method must not be non-static without a target");}ReflectionUtils.makeAccessible(preparedMethod);return preparedMethod.invoke(targetObject, getArguments());}

Method的invoke方法如下，

1）如果是覆写的方法，调用Reflection.getCallerClass()的native方法获取调用者的类，并对调用者的类和目标对象进行检查；2）获取MethodAccessor对象，如果当前Method没有，则在根节点root不为null时从根节点获取MethodAccessorImpl对象，否则调用反射工厂的newMethodAccessor；3）调用MethodAccessorImpl对象（MethodAccessor接口的实现类）的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 volatileif (ma == null) {ma = acquireMethodAccessor();}return ma.invoke(obj, args);}

@CallerSensitive注解作用：

jvm的开发者认为Reflection.getCallerClass()方法危险，不希望开发者调用，就把这种危险的方法用 @CallerSensitive修饰，并在JVM级别检查，参考文末链接3。

ReflectionFactory的newMethodAccessor方法如下，其中isAnonymousClass方法检查基础类是否为匿名类。

public MethodAccessor newMethodAccessor(Method var1) {checkInitted();if (noInflation && !ReflectUtil.isVMAnonymousClass(var1.getDeclaringClass())) {return (new MethodAccessorGenerator()).generateMethod(var1.getDeclaringClass(), var1.getName(), var1.getParameterTypes(), var1.getReturnType(), var1.getExceptionTypes(), var1.getModifiers());} else {NativeMethodAccessorImpl var2 = new NativeMethodAccessorImpl(var1);DelegatingMethodAccessorImpl var3 = new DelegatingMethodAccessorImpl(var2);var2.setParent(var3);return var3;}}

参考链接：

1、Java ReflectionTestUtils.invokeMethod方法代码示例 - 纯净天空

2、LeetCode - Medium - 332. Reconstruct Itinerary-蒲公英云

3、JEP 176: Mechanical Checking of Caller-Sensitive Methods

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