Java reflection Get class and object information fully resolved

Reflection resolves an issue in which objects and classes are not predictable at compile time, and information about the object and class is not known until the program runs.

When two people collaborate on development, you can do the initial development by knowing the other person's class name.

Get Class object

• Class.forName (String clazzname) static method
• invokes the class property of the Person.class, which returns the person's class object (recommended)
• Call the GetClass () method of an object

Specific use or to choose according to the actual, the first way is relatively free, as long as you know a class name can be, it does not do the existence of the check, the second, the third will do the check

Get information about a class get class Builder

• Connstructor<T> getConstructor(Class<?>...parameterTypes): Returns the public constructor with the specified parameters for the class of this class object
• Constructor<?>[] getConstructors(): Returns all public constructors for the class of this class object
• Constructor<T>[] getDeclaredConstructor(Class<?>...parameterTypes): Returns the constructor with the specified parameters for the class of the classes object, independent of the constructor's access rights
• Constructor<?>[] getDeclaredConstructors(): Returns all constructors for classes of this class object, regardless of the constructor's access rights

Get class member methods

• Method getMethod(String name,Class<?>...parameterTypes): Returns the public method with the specified parameters for the class of this class object
• Method[] getMethods(): Returns all public methods of the class represented by this class object
• Method getDeclaredMethod(string name,Class<?>...parameterTypes): Returns the method with the specified parameter for the class of this class object, regardless of the method access permission
• Method[] getDeclaredMethods(): Returns all the methods of the class object corresponding to the access rights of the method

Get class member variable

• Field getField(String name): Returns the public member variable for the specified name of the class for this class object
• Field[] getFields(): Returns all public member variables for the class of this class object
• Field getDeclaredField(String name): Returns the member variable of the specified name of the class object corresponding to the member variable access permission
• Field[] getDeclaredFields(): Returns all member variables of the class object corresponding to the access rights of the member variable

Get class annotations

• <A extends Annotation>A getAnnotation(Class<A>annotationClass): Attempts to obtain the annotation of the specified type of the village on the class object, and returns null if the type annotation does not exist
• <A extends Annotation>A getDeclaredAnnotation(Class<A>annotationClass): This is new in Java 8, which gets the annotation of the specified type that directly modifies the class of that class object, and returns null if it does not exist.
• Annotation[] getAnnotations(): Returns all annotation that exist on the class object corresponding to
• Annotation[] getDeclaredAnnotations(): Returns all annotation that exist on the class object corresponding to
• <A extends Annotation>A[] getAnnotationByType(Class<A>annotationClass): The function of this method is basically similar to the Getannotation () method described earlier, but because Java8 adds a repeating annotation feature, you need to use this method to get more than one annotation that modifies the specified type of the class.
• <A extends Annotation>A[] getDeclaredAnnotationByType(Class<A>annotationClass): This method is similar to the Getdeclaredannotations () method described earlier, and is also because of the repetitive annotation function of Java8, which requires the method to obtain multiple annotation that directly decorate the specified type of the class.

Gets the class inner class

• Class<?>[] getDeclaredClasses(): Returns all the inner classes contained in the class formation corresponding classes

Gets the outer class where the class object resides

• Class<?> getDeclaringClass(): Returns the outer class where the class object corresponds

Gets the interface implemented by the class for the class object

• Class<?>[] getInterfaces(): Returns all interfaces implemented by the class object's corresponding classes

Gets the parent class for which the class object class inherits

• Class<? super T> getSuperclass(): The class object that returns the superclass of the class object

Gets the basic information about the class's modifier, package, class name, and so on for the class object

• int getModifiers(): Returns all the modifiers for this class or interface, which consist of the constants of public, protected, private, final, static, abstract, and so on, and the returned integers should be decoded using the method of the Modifier tool class. To get the real modifier.
• Package getPackage(): Gets the package for this class
• String getName(): Returns the short name of the class represented by this class object as a string.

Determines whether the class is an interface, enumeration, annotation type

• boolean isAnnotation(): Returns whether this class object represents an annotation type
• boolean isAnnotationPresent(Class<? extends Annotation>annotationClass): Determines whether this class object uses the annotation adornment
• boolean isAnonymousClass(): Returns whether this class object is an anonymous class
• boolean isArray(): Returns whether this class object represents an array class
• boolean isEnum(): Returns whether this class object represents an enumeration
• boolean isInterface(): Returns whether this class object represents an interface
• boolean isInstance(Object obj): Determines if obj is an instance of this class object, which can completely replace the instanceof operator

Public interface colorable {public     void value ();}

public class ClassInfo {public    static void Main (string[] args) throws Nosuchmethodexception, SecurityException {
   
    class<colorable> Cls=colorable.class;        System.out.println (Cls.getmethod ("value"));        System.out.println (Cls.isannotation ());        System.out.println (Cls.isinterface ());}    }
   

结果

public abstract void Com.em.Colorable.value () falsetrue

New method parameter Reflection in Java8

• int getParameterCount(): Gets the number of parameters for the constructor or method
• Parameter[] getParameters(): Gets all the parameters of the constructor or method
• getModifiers(): Gets the modifier that modifies the parameter
• String getName(): Get formal parameter names
• Type getParameterizedType(): Gets the type of the parameter with the generic type
• Class<?>getType(): Getting formal parameter types
• boolean isNamePresent(): This method returns whether the class file of the classes contains the parameter name information for the method
• boolean isVarArgs(): This method is used to determine whether the parameter is a variable number of formal parameters

public class Test {public    void GetInfo (String str,list<string>list) {        System.out.println ("Success");}    }

public class ClassInfo {public    static void Main (string[] args) throws Nosuchmethodexception, SecurityException {
   
    class<test> Cls=test.class;        Method Med=cls.getmethod ("GetInfo", string.class,list.class);        System.out.println (Med.getparametercount ());        Parameter[] Params=med.getparameters ();        System.out.println (params.length);        for (Parameter par:params) {            System.out.println (Par.getname ());            System.out.println (Par.gettype ());            System.out.println (Par.getparameterizedtype ());}}}    
   

结果

22arg0class Java.lang.Stringclass Java.lang.Stringarg1interface java.util.listjava.util.list<java.lang.string >

Reflection-generated objects

• Use the Newinstance () method of the class object to create an instance of the class object that requires a default constructor (more commonly used)
• Use the class object first to get the specified constructor object, and in the call to the constructor object's Newinstance () method to create an instance of the class object corresponding to

Reflection Call Method

• Object invoke(Object obj,Object...args): Obj In this method is the keynote that executes the method, and the following args is the argument that is passed in when the method is executed

public class Test {public    Test (String str) {        System.out.println (str);    }    public void GetInfo (String str) {        System.out.println (str);}    }

public class ClassInfo {public    static void Main (string[] args) throws Exception {        class<test> cls=test.cl the;        Constructor<test>construct=cls.getconstructor (string.class);        Test test=construct.newinstance ("Initialize");        Method Med=cls.getmethod ("GetInfo", string.class);        Med.invoke (Test, "call method succeeded");}    }

结果

Initialize Call method succeeded

Next crossing look closely at the chestnuts below

public class Test {public    Test (String str) {        System.out.println (str);    }    Proprietary method Private    void GetInfo (String str) {        System.out.println (str);}    }

public class ClassInfo {public    static void Main (string[] args) throws Exception {        class<test> Cls=test.cla SS;        Constructor<test>construct=cls.getconstructor (string.class);        Test test=construct.newinstance ("Initialize");      Why use this method?        Method Med=cls.getdeclaredmethod ("GetInfo", string.class);      Why use this method?        med.setaccessible (true);        Med.invoke (Test, "call method succeeded");}    }

结果

Initialize Call method succeeded

Setaccessible (Boolean flag): Sets the value to true to indicate that the method is in use and that the Java language's access check should be canceled

accessing member variable values

• getXxx(Object obj): Gets the value of this member variable for the Obj object. Here XXX corresponds to 8 basic types, if the type of the member variable is a reference type, then remove the xxx part
• setXxx(Object obj,Xxx val): Sets the member variable of the Obj object to a Val value. The xxx here corresponds to the basic type in 8, and if the type of the member variable is a reference type, then the XXX after the set is canceled

The above two methods can be method for all member variables, including private member variables

public class Test {    private int num;    Public Test (String str) {        System.out.println (str);    }    private void GetInfo (String str) {        System.out.println (str);    }    public int Getnum () {        return num;    }    public void setnum (int num) {        this.num = num;    }}

public class ClassInfo {public    static void Main (string[] args) throws Exception {        class<test> Cls=test.cla SS;        Constructor<test>construct=cls.getconstructor (string.class);        Test test=construct.newinstance ("Initialize");        Method Med=cls.getdeclaredmethod ("GetInfo", string.class);        Med.setaccessible (true);        Med.invoke (Test, "call method succeeded");        Field fld=cls.getdeclaredfield ("num");        Fld.setaccessible (true);        Fld.setint (test,);        System.out.println (Fld.getint (test));}    }

结果

Initialize Call method succeeded 12

manipulating arrays

There is an array class under the Java.lang.reflect package that can dynamically create an array

static Object newInstance(Class<?>componentType,int...length): Creates a new array with the specified element type, specified dimension

static xxx getXxx(Object array,int index): Returns the index element of the array. where xxx is a variety of basic data types, if the array element is a reference type, then the method becomes get ()

static void setXxx(Object array,int index,xxx val): Sets the value of a low index element in the array array to Val, where xxx is a variety of base data types, and if the array element is a reference type, the method becomes set ()

public class ArrayInfo {public    static void Main (string[] args) {        Object arrays=array.newinstance (String.class, 3);        Array.set (arrays, 0, "the first One");        Array.set (arrays, 1, "the second One");        Array.set (Arrays, 2, "third");        System.out.println (Array.get (arrays, 2));}    }

Java reflection Get class and object information fully resolved