08.Java Basics - Reflection-javaTutorial-php.cn

Basic concepts

##Java reflection mechanism allows us to )

Outside of the runtime (Runtime) Check the information of classes, interfaces, variables and methods. The functions of reflection are as follows:

For any class, you can know all the properties and methods of this class;

For any object, you can call any of its methods and properties.

Class

#To obtain information about a class, you first need to obtain the Class object of the class.

All types in Java include basic types (int, long, float, etc.), even arrays have Class objects associated with them.

//1.在编译期获取类对象Class cls = Demo.class 

//2.在运行期获取类对象（注意：需要包括完整的包名） Cllss clas = Class.forName("com.test.Demo")

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Class cls = ...// 1.获取完整类型（包括包名）String className = cls.getName();
// 2.获取简单类名 String simpleClassName =cls.getSimpleName();

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Class cls = ...// 修饰符都被包装成一个 int 类型的数字，这样每个修饰符都是一个位标识(flag bit)    
int modifiers  = cls.getModifiers();// 判断修饰符类型Modifier.isPublic( modifiers);
Modifier.isProtected( modifiers);
Modifier.isPrivate( modifiers);
Modifier.isAbstract( modifiers);
Modifier.isInterface( modifiers);
Modifier.isFinal( modifiers);
Modifier.isStatic( modifiers);
Modifier.isNative( modifiers);
Modifier.isStrict( modifiers);
Modifier.isSynchronized( modifiers);
Modifier.isTransient( modifiers);
Modifier.isVolatile( modifiers);

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Class cls =...
Package p = cls.getPackage();

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Class cls = ...
Class superClass  = cls.getSuperclass();

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Class cls = ...// 类可以实现多个接口，因此返回的是接口数组Class [ ] interfaces = cls.getInterfaces();

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Constructor

In Java, the Constructor class represents a constructor object.

Get

Constructor method

Class cls = ...// 1.获取所有构造函数Constructor[] constructors = cls.getConstructors(); 

// 2.获取指定构造函数，不存在匹配则抛出异常。
// 对应的构造函数入参类为 Person(String str, int i)Constructor constructor = 
    cls.getConstructor(new Class[]{String.class,int.class});

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Constructor constructor = ...
// 因为构造函数的入参有多个，因此返回的数组Class [] paramterTypes = constructor.getParameterTypes();
for(Class paramterType: paramterTypes ){    
// do something...}

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Class cls = ...
Constructor constructor = cls.getConstructor(new Class[]{String.class});
// Object[] 数组表示入参的具体值，需要与 Class[] 的入参类型完全匹配Demo demo = (Demo) constructor.newInstance(new Object[]{"hello"});

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Field

In Java, the Field class represents a variable object.

Class cls = ...// 1.获取所有公共变量Field[] fields = cls.getFields();
// 2.获取所有非公共变量Field [] privateFields =cls.getDeclaredFields();

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Class cls = ...//入参为变量名称，名称不匹配则抛出异常// 获取指定公共变量Field field = cls.getField("num");
// 获取指定非公共变量Field privateField = cls.getDeclaredField("str");

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Class<?> cls = Demo.class;
Demo demo = (Demo) cls.newInstance();// 1.1.公共方法的 setter、getterField field = cls.getField("num");
field.set(demo, 1000);int filedValue  = (Integer) field.get(demo);// 2.1 非公共方法的 setter、getterField privateField = cls.getDeclaredField("str");
privateField.setAccessible(true); // 关键 -> 操作之前，需要设置访问权限privateField.set(demo, "hello");
String privateFieldValue = (String) privateField.get(demo);

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Method

1. Get the method object

Class cls = ...// 1.获取所有公共方法（包括从父类继承而来的）Method[] methods = cls.getMethods();
// 2.获取所有非公共方法（包括从父类继承而来的）Method [] privateMethods = cls.getDeclaredMethods();

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Class cls = ...// 1.获取指定公共方法，入参为：方法名、方法入参Method method = cls.getMethod("print", new Class[]{String.class});
// 2.获取指定非公共方法，入参为：方法名、方法入参Method privateMethod = cls.getDeclaredMethod("say", new Class[]{int.class});

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Method method = ...// 1.获取方法的所有入参类型Class [] paramterTypes = method.getParameterTypes();
// 2.获取方法的返回值类型Class reeturnType = method.getReturnType();

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Class cls = Demo.class;
Demo demo = (Demo) cls.newInstance();
Method method = cls.getMethod("print", new Class[]{String.class});//1.执行公用方法，入参为：类实例，方法入参的具体值method.invoke(demo, "hello");
//2.执行非公共方法Method privateMethod = 
    cls.getDeclaredMethod("print", new Class[]{String.class});
privateMethod.setAccessible(true); // 关键 -> 操作之前，需要获得访问权限privateMethod.invoke(demo, "hello");

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public static boolean isGetter(Method method){    
if(!method.getName().startsWith("get")){        
return false;
    }    
    if(method.getParameterTypes().length !=0){        
    return false;
    }    
    if(void.class.equals(method.getReturnType())){        
    return false;
    }    
    return true;
}
public static boolean isSetter(Method method){    
if(!method.getName().startsWith("set")){        
return false;
    }    
    if(method.getParameterTypes().length !=1){        
    return false;
    }    
    return true;
}

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Array

In Java, use the Array class Represents an array object.

int [ ] intArray = ... 
Class arrayCls = intArray.class;

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Class arrayCls = ...
Class arrayComponentType =arrayCls .getComponentType();

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// 创建一个数组，类型为 int ，容量为 3int [ ] intArray = (int[]) Array.newInstance(int.class, 3);
//在 JVM 中字母I代表int类型，左边的‘[’代表我想要的是一个int类型的数组Class intArrayClass = Class.forName("[I");  

//注意‘[L’的右边是类名，类名的右边是一个‘;’符号。这个的含义是一个指定类型的数组。
 stringArrayClass = Class.forName("[Ljava.lang.String;");

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int [ ] intArray = ...// 添加元素，数组为 intArray，位置为 0 ，值为 100Array.set(intArray, 0, 100);
Array.set(intArray, 1, 200);
System.out.println("intArray[0] = " + Array.get(intArray, 0));
System.out.println("intArray[1] = " + Array.get(intArray, 1));

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