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)