Java--reflect reflection mechanism

• The use of class classes
  • In the world of OOP, everything is object. Whose object is the class? --The class is an object, and the class is an instance object of the Java.lang.Class class! there is a class named class.
  • Instance:

  • //there is a class ofclassFoo {voidPrint () {System.out.println ("Foo"));} }//in the main function: Public Static voidMain (string[] args) {//Foo This class is also an instance object, the Class class instance object, how to express it//Any class is an instance object of class, and this instance object is represented in three ways//1.--actually tells us that any class has an implicit static member variable-classClass C1 = Foo.class; //2. Objects of this class are known to pass the GetClass methodFoo F1 =NewFoo (); Class C2=F1.getclass (); //C1, C2 represents the class type of the Foo class ( class type)//A class is also an object, an instance object of class, which becomes the class type of the class.//whether from C1,C2 represents the class type of the Foo class, a class can only be an instance object of classSYSTEM.OUT.PRINTLN (C1 = = C2);//true//3.Class C3 =NULL; Try{C3= Class.forName ("Com.immoc.reflect.Foo"); } Catch(ClassNotFoundException e) {//TODO auto-generated Catch blockE.printstacktrace (); } System.out.println (C3= = C2);//true        /** We can create an object instance from the class type of the class; * Create an object of the Foo class by C1 or C2 or C3*/        Try{foo Foo= (Foo) c1.newinstance ();//requires an argument-free construction methodFoo.print (); } Catch(instantiationexception e) {//TODO auto-generated Catch blockE.printstacktrace (); } Catch(illegalaccessexception e) {//TODO auto-generated Catch blockE.printstacktrace (); }    }    

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  • Dynamic load classes: For example, Class.forName ("Full name of the class") not only represents the class type of the class, but also represents the dynamic load class
    • Compile time load class is static load class, runtime load class is dynamically loaded
    • The new object is a statically loaded class that needs to load all the classes that may be used at compile time
    • Dynamic load classes:
      • Class C = class.forname (Args[0]);
      • Officeable OA = (officeable) c.newinstance (); To create an object instance of this class from a class type
    • Functional classes use dynamic loading as much as possible, rather than static loading
  • Basic data type, void keyword exists class type
  • Basic API operations for class classes

  • /*** Print class information, including member functions of the class, member variables *@paramobj*/     Public Static voidprintclassmessage (Object obj) {//to get information about a class, you first get the class type of the classClass C = Obj.getclass ();//The object of which subclass is passed, and C is the class type of the classSystem.out.println ("The name of the class is:" +c.getname ()); /** Methods Class, Method Object * A member method is a Methods object * the GetMethod () method obtains all public functions, including those inherited by the parent class .*/method[] Ms=C.getmethods ();  for(Method m:ms) {//gets the class type of the method return value typeClass ReturnType =M.getreturntype (); System.out.print (Returntype.getname ()+ " "); System.out.print (M.getname ()+ "("); //get parameter Type--Get the class type of the type of the parameter listclass[] Paramtypes =m.getparametertypes ();  for(Class p:paramtypes) {System.out.print (P.getname ()+ ","); } System.out.println (")"); }    }

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• Reflection of the method

  •  Public Static voidprintmethodmessage (Object obj) {//to get information about a class, you first get the class type of the classClass C = Obj.getclass ();//The object of which subclass is passed, and C is the class type of the classSystem.out.println ("The name of the class is:" +c.getname ()); /** Methods Class, Method Object * A member method is a Methods object * the GetMethod () method obtains all public functions, including those inherited by the parent class .*/method[] Ms=C.getmethods ();  for(Method m:ms) {//gets the class type of the method return value typeClass ReturnType =M.getreturntype (); System.out.print (Returntype.getname ()+ " "); System.out.print (M.getname ()+ "("); //get parameter Type--Get the class type of the type of the parameter listclass[] Paramtypes =m.getparametertypes ();  for(Class p:paramtypes) {System.out.print (P.getname ()+ ","); } System.out.println (")"); }    }

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  • How to get a method
    • The name of the method and the parameter list of the method can only determine a method
  • Action reflected by the method
    • Method.invoke (object, parameter list)

    •  Public classMethodDemo1 { Public Static voidMain (string[] args) {/** To get the print (int, int) method * First get information about the class (class information of Class)*/A A1=NewA (); Class C=A1.getclass (); //2. Get the method, name and argument list        Try{Method m= C.getdeclaredmethod ("Print",Newclass[]{int.class,int.class}); //Method Reflection Operation: Invoking a method with an objectM.invoke (A1,NewOBJECT[]{10, 20}); } Catch(Exception e) {//TODO auto-generated Catch blockE.printstacktrace (); }             }}classA { Public voidPrintintAintb) {System.out.println (a+b); }     Public voidPrint (String A, string b) {System.out.println (A.touppercase ()+ "," +b.tolowercase ()); }}

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• Reflection of member variables
  • Member variables are also objects, Java.lang.reflect.Field, Field classes encapsulate operations on member variables
  • The GetFields () method gets the information for all public member variables
  • Getdeclaredfields () Gets the information for all member variables declared by the class itself

  • //first get the type, and then get the name of the variable://such as: private int t; Public Static voidprintfieldmessage (Object obj) {System.out.println ("---------------------------------------------"); Class C=Obj.getclass (); Field[] FS=C.getdeclaredfields ();  for(Field f:fs) {//the class type that gets the type of the member variable: intClass FieldType =F.gettype (); String TypeName=Fieldtype.getname (); //get the name of the member variable: TString FieldName =F.getname (); System.out.println ("TypeName:" + typeName + "FieldName:" +fieldName); }    }

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• Reflection of the constructor function

  • /*** Print constructor information (constructor is also object: java.long.Constructor) *@paramobj*/     Public Static voidprintconmessage (Object obj) {Class C=Obj.getclass (); Constructor[] CS=c.getdeclaredconstructors ();  for(Constructor con:cs) {System.out.print (Con.getname ()+ "("); //Get parameter listclass[] Paramtypes =con.getparametertypes ();  for(Class cl:paramtypes) {System.out.print (Cl.getname ()+ ","); } System.out.println (")"); }    }

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• Java class loading mechanism
• Understanding the nature of set generics through reflection
  • Generics in a collection in Java are prevented from being entered incorrectly, only in the compile phase, and not after compilation. Validation: Reflection by method

  •  Public Static voidMain (string[] args) {//TODO auto-generated Method StubArrayList L1 =NewArrayList (); ArrayList<String> L2 =NewArraylist<string>(); L1.add ("Hello"); L1.add (2);//wrong? Is it related to the JDK version??? Class C1 =L1.getclass (); Class C2=L2.getclass (); SYSTEM.OUT.PRINTLN (C1= = C2);//true to explain that the generics of the compiled collection are de-generalized//reflected operations are post-compilation operations                Try{Method m= C2.getmethod ("Add", Object.class); M.invoke (L1,"World");        System.out.println (L1.size ()); } Catch(Exception e) {//TODO auto-generated Catch blockE.printstacktrace (); }     }

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Application: The name of the class is used to produce an object such as: integer in = (integer) class.forname (className). newinstance ();

Java--reflect reflection mechanism