我看JAVA 之 Class

注：基于jdk11

Class

类 Class 的实例代表运行时java应用程序中的类和接口。枚举类型是一种类，注解类是一种接口。数组是共享同一种类的。基 本类型（boolean、byte、char、short、int、long、float和double）和关键字void也表示为类对象。 Class 没有公开的构造方法，只能由JVM创建。 当类加载器调用defineClass方法并传递类文件字节给jvm，jvm会自动构建 Class 对象。

实现了如下接口

1. java.io.Serializable 2. GenericDeclaration GenericDeclaration 所有声明泛型变量的实体要实现此接口（注：后续会在“我看JAVA 之 泛型“阐述） 唯一的方法： public TypeVariable<?>[] getTypeParameters();//获取当前“实体”上声明的“泛型变量"。 3. Type Type是java语言中所有类型的公共高级接口，它们包括原始类型、参数化类型、数组类型、泛型变量和基本类型。 默认方法： default String getTypeName() { return toString(); } 4. AnnotatedElement java.lang.reflect.AnnotatedElement 接口定义了获取在当前JVM中运行程序的注解元素。这个接口允许通过反射的方式获取注解。通过当前接口方法获取到的注解都是不可变且序列化的。 

几个重点方法

1. forName()

 public static Class<?> forName(String className) throws ClassNotFoundException { Class<?> caller = Reflection.getCallerClass(); return forName0(className, true, ClassLoader.getClassLoader(caller), caller); } @CallerSensitive public static Class<?> forName(String name, boolean initialize, ClassLoader loader) throws ClassNotFoundException { Class<?> caller = null; SecurityManager sm = System.getSecurityManager(); if (sm != null) { // Reflective call to get caller class is only needed if a security manager // is present. Avoid the overhead of making this call otherwise. caller = Reflection.getCallerClass(); if (loader == null) { ClassLoader ccl = ClassLoader.getClassLoader(caller); if (ccl != null) { sm.checkPermission( SecurityConstants.GET_CLASSLOADER_PERMISSION); } } } return forName0(name, initialize, loader, caller); } /** Called after security check for system loader access checks have been made. */ private static native Class<?> forName0(String name, boolean initialize, ClassLoader loader, Class<?> caller) throws ClassNotFoundException;

几个获取 Class 对象引用方式

1. Integer.class 2. new Integer(1).getClass() 3. Class.forName("java.lang.Integer") 

代码示例

 package chapter03; import java.lang.annotation.ElementType; import java.lang.reflect.InvocationTargetException; public class TestClass { public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException { System.out.println("--- 3 mode get obj's Class ---"); System.out.println(Integer.class); System.out.println(Integer.valueOf(1).getClass()); System.out.println(Class.forName("java.lang.Integer")); System.out.println(int.class); System.out.println("int is privitive type or not ? " + int.class.isPrimitive()); // array System.out.println("--- array' Class ---"); System.out.println(new Integer[]{1, 3, 5}.getClass()); System.out.println(new Object[]{1, 3, 5}.getClass()); System.out.println(new Integer[][]{{1, 2, 3}, {2, 3, 4}, {3, 4, 5}}.getClass()); // void package System.out.println("--- void & package' Class ---"); System.out.println(Void.class); System.out.println(void.class); System.out.println(Package.class); // enum System.out.println("--- enum' Class ---"); System.out.println(ElementType.class); System.out.println("ElementType is enum or not ? " + ElementType.class.isEnum()); // annotation System.out.println("--- annotation' Class ---"); System.out.println(Deprecated.class); System.out.println("Deprecated is interface or not ? " + Deprecated.class.isInterface()); System.out.println("Deprecated is annotation or not ? " + Deprecated.class.isAnnotation()); // class' Class System.out.println("--- class' Class ---"); System.out.println(Class.forName("java.lang.Integer").getClass()); // module System.out.println(Class.forName("chapter03.Test").getModule()); //new instance System.out.println(Class.forName("chapter03.Test").getDeclaredConstructor()); Class.forName("chapter03.Test").getDeclaredConstructor().newInstance(); Class.forName("chapter03.Test").newInstance(); } } class Test { private Test() { } } 打印结果： --- 3 mode get obj's Class --- class java.lang.Integer class java.lang.Integer class java.lang.Integer int int is privitive type or not ? true --- array' Class --- class [Ljava.lang.Integer; class [Ljava.lang.Object; class [[Ljava.lang.Integer; --- void & package' Class --- class java.lang.Void void class java.lang.Package --- enum' Class --- class java.lang.annotation.ElementType ElementType is enum or not ? true --- annotation' Class --- interface java.lang.Deprecated Deprecated is interface or not ? true Deprecated is annotation or not ? true --- class' Class --- class java.lang.Class unnamed module @67b64c45 private chapter03.Test() Exception in thread "main" java.lang.IllegalAccessException: class chapter03.TestClass cannot access a member of class chapter03.Test with modifiers "private" at java.base/jdk.internal.reflect.Reflection.newIllegalAccessException(Reflection.java:361) at java.base/java.lang.reflect.AccessibleObject.checkAccess(AccessibleObject.java:591) at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:481) at chapter03.TestClass.main(TestClass.java:50)