Java反射机制 - 日理万妓 2022-03-06 01:48 222阅读 0赞 Class对象 虚拟机在class文件的加载阶段,把类信息保存在方法区数据结构中,并在Java堆中生成一个Class对象,作为类信息的入口。 声明两个类,Cat.java 和 Dog.java class Cat \{ private String name; private int age; static \{ System.out.println("Cat is load"); \} \} class Dog \{ private String name; private int age; static \{ System.out.println("Dog is load"); \} \} 获取Class对象一般有三种方式: 通过实例变量方式 public class test \{ public static void main(String\[\] args) \{ Dog dog = new Dog(); Class clazz = dog.getClass(); \} \} public class test \{ public static void main(String\[\] args) \{ Dog dog = new Dog(); Class clazz = dog.getClass(); \} \} 通过类名方式 public class test \{ public static void main(String\[\] args) \{ Class clazz = Dog.class; \} \} public class test \{ public static void main(String\[\] args) \{ Class clazz = Dog.class; \} \} 通过这种方式时,只会加载Dog类,并不会触发其类构造器的初始化。 通过Class.forName(String classname)方式 public class ClassTest \{ public static void main(String\[\] args) \{ try \{ Class clazz = Class.forName("zzzzzz.Dog"); \} catch (ClassNotFoundException e) \{\} \} \} public class ClassTest \{ public static void main(String\[\] args) \{ try \{ Class clazz = Class.forName("zzzzzz.Dog"); \} catch (ClassNotFoundException e) \{\} \} \} 在JDK源码实现中,forName方法会调用Native方法forName0(),它在JVM中调用findClassFromClassLoader()加载Dog类,其原理和ClassLoader一样,将会触发Dog类的类构造器初始化,forName0方法声明如下: private static native Class<?> forName0(String name, boolean initialize, ClassLoader loader, Class> caller) 1 private static native Class<?> forName0(String name, boolean initialize, ClassLoader loader, Class> caller) 其中initialize参数,用来告诉虚拟机是否需要对加载的类进行初始化,如果initialize为false,则不会进行初始化Dog类。 Class clazz = Class.forName("zzzzzz.Dog", false, Dog.class.getClassLoader()); 1 Class clazz = Class.forName("zzzzzz.Dog", false, Dog.class.getClassLoader()); 反射机制 反射机制reflect可以在运行期间获取类的字段、方法、父类和接口等信息。 1、获取类字段 Class class\_dog = Dog.class; Field\[\] fields = class\_dog.getDeclaredFields(); for (Field field : fields) \{ System.out.println(field.getName()); \} 1 Class class\_dog = Dog.class; 2 Field\[\] fields = class\_dog.getDeclaredFields(); 3 for (Field field : fields) \{ 4 System.out.println(field.getName()); 5 \} 2、获取类方法 Class class\_dog = Dog.class; Method\[\] methods = class\_dog.getDeclaredMethods(); for (Method method : methods) \{ System.out.println(method); \} Class class\_dog = Dog.class; Method\[\] methods = class\_dog.getDeclaredMethods(); for (Method method : methods) \{ System.out.println(method); \} 通过method.invoke(obj, ...args)可以调用obj实例的method方法。 3、获取对应的实例构造器,并生成类实例 public class ClassTest \{ public static void main(String\[\] args) throws NoSuchMethodException \{ Class class\_dog = Dog.class; Constructor constructor = class\_dog.getConstructor(String.class, int.class); constructor.newInstance("Tom", 10); \} \} class Dog \{ private String name; private int age; public Dog(String name, int age) \{ this.name = name; this.age = age; \} \} public class ClassTest \{ public static void main(String\[\] args) throws NoSuchMethodException \{ Class class\_dog = Dog.class; Constructor constructor = class\_dog.getConstructor(String.class, int.class); constructor.newInstance("Tom", 10); \} \} class Dog \{ private String name; private int age; public Dog(String name, int age) \{ this.name = name; this.age = age; \} \} 如果没有显示的声明默认构造器,class\_dog.getConstructor()会抛出NoSuchMethodException异常。 4、通过newInstance()方法生成类实例 Class class\_dog = Dog.class; Dog dog = class\_dog.newInstance(); Class class\_dog = Dog.class; Dog dog = class\_dog.newInstance(); 5、设置私有变量 Class class\_dog = Dog.class; Field name = class\_dog.getDeclaredField("name"); name.setAccessible(true); Dog dog = (Dog) class\_dog.newInstance(); name.set(dog, "Tom"); Class class\_dog = Dog.class; Field name = class\_dog.getDeclaredField("name"); name.setAccessible(true); Dog dog = (Dog) class\_dog.newInstance(); name.set(dog, "Tom"); 6、获取私有变量 Field f = Unsafe.class.getDeclaredField("theUnsafe"); f.setAccessible(true); return (Unsafe)f.get(null); Field f = Unsafe.class.getDeclaredField("theUnsafe"); f.setAccessible(true); return (Unsafe)f.get(null); 这种方式在使用Unsafe类进行黑魔法时经常用到。 反射的性能问题 Stackoverflow上,很多人觉得使用反射reflect会影响系统性能,主要有以下几点看法: 1、代码的验证防御逻辑过于复杂,本来这块验证时在链接阶段实现的,使用反射reflect时需要在运行时进行; 2、产生过多的临时对象,影响GC的消耗; 3、由于缺少上下文,导致不能进行更多的优化,如JIT; 不过现代JVM已经运行的足够快,我们应该把主要重心放在复杂的代码逻辑上,而不是一开始就进行各种性能优化。
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