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
- 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)
