java reflection mechanism
- 2020-06-03 06:23:44
- OfStack
Guidance of this paper:
By reflection mechanism
Gets basic information about the class Gets the annotation information for the class Get generic information
package reflection;
@AnnotationUserTable("datebaseExample")
public class User {
@AnnotationUserField(uName="name",type="varchar",length=10)
private String name;
@AnnotationUserField(uName="age",type="int",length=3)
private int age;
@AnnotationUserField(uName="sex",type="char",length=2)
private String sex;
public User() {
super();
}
public User(String name, int age, String sex) {
super();
this.name = name;
this.age = age;
this.sex = sex;
}
public String getName() {
return name;
}
public void setName() {
this.name = "test";
}
public int getAge() {
return age;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
}
bean:User
package reflection;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(value={ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface AnnotationUserTable {
String value();
}
Custom annotations: Class annotations
package reflection;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(value={ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface AnnotationUserField {
String uName();
String type();
int length();
}
Custom annotations: Attribute annotations
package reflection;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
public class Demo01 {
static Class<?> c = null;
public static void main(String[] args) {
try {
c = Class.forName("reflection.User");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
test();// Gets information about a class's properties, methods, and so on
}
static void test(){
try {
// Gets the name of the class
System.out.println(" Gets the name of the class ");
System.out.println("getName():" + c.getName());// To get the package name + The name of the class
System.out.println("getSimpleName():" + c.getSimpleName());// Get the name of the class
System.out.println("getCanonicalName():" + c.getCanonicalName());// Get the name of the class
System.out.println("*******************************");
// Get attribute information
System.out.println(" Get attribute information ");
Field[] fields = c.getDeclaredFields();
// Field[] fields = c.getFields(); Can only get public Modifies attribute information
for (Field f : fields) {
String fName = f.getName();
System.out.println(c.getDeclaredField(fName));
}
System.out.println("*******************************");
// Get method information
System.out.println(" Get method information ");
Method[] methods = c.getDeclaredMethods();
for (Method m : methods) {
// String mName = m.getName();
System.out.println(m.getName() + "-->" + m);
}
System.out.println(" Get the corresponding by name alone getName methods :" + c.getDeclaredMethod("getName"));
System.out.println(" Get the corresponding by name alone setSex methods :" + c.getDeclaredMethod("setSex", String.class));// Method has arguments and must pass the parameter type
System.out.println("*******************************");
// Gets constructor information
System.out.println(" Gets constructor information ");
Constructor<?>[] constructor = c.getConstructors();
for (Constructor<?> cons : constructor) {
System.out.println(cons);
}
} catch (NoSuchFieldException | SecurityException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
}
}
}
main1 : Gets basic information about a class through a reflection mechanism
output:
Gets the name of the class
getName():reflection.User
getSimpleName():User
getCanonicalName():reflection.User
*******************************
Get attribute information
private java.lang.String reflection.User.name
private int reflection.User.age
private java.lang.String reflection.User.sex
*******************************
Get method information
getName-->public java.lang.String reflection.User.getName()
setName-->public void reflection.User.setName()
setSex-->public void reflection.User.setSex(java.lang.String)
getSex-->public java.lang.String reflection.User.getSex()
getAge-->public int reflection.User.getAge()
Get the corresponding by name alone getName methods :public java.lang.String reflection.User.getName()
Get the corresponding by name alone setSex methods :public void reflection.User.setSex(java.lang.String)
*******************************
Gets constructor information
public reflection.User()
public reflection.User(java.lang.String,int,java.lang.String)
View Console
In the following example, annotation information of a class is obtained through reflection mechanism.
package reflection;
import java.lang.reflect.Field;
/**
* Gets information about a class's properties, methods, and so on
* 1. Get the element object (such as the attribute) (Note: read the class annotation, which seems to be less 1 Step)
* 2. Gets the annotation object of the specified type of the element object
* 3. Read the corresponding value of the annotation object
*/
public class Test02 {
static Class<?> c = null;
public static void main(String[] args) {
try {
c = Class.forName("reflection.User");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
test();
}
static void test(){
try {
// Gets the specified annotation for the class
System.out.println("*********** The specified annotation of the class **************");
AnnotationUserTable table = (AnnotationUserTable)c.getAnnotation(AnnotationUserTable.class);
System.out.println(table.value());
// Gets the specified annotation for the property
System.out.println("*********** Attribute *************");
Field field = c.getDeclaredField("name");
AnnotationUserField annoField = (AnnotationUserField)field.getAnnotation(AnnotationUserField.class);
System.out.println(annoField.uName()+"\t"+annoField.type()+"\t"+annoField.length());
// Spell out the table name and field information obtained DDL Statement, and then pass JDBC Connect to database query
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (SecurityException e) {
e.printStackTrace();
}
}
}
output:
*********** The specified annotation of the class **************
datebaseExample
*********** Attribute *************
name varchar 10
In the following example, generic information is obtained through reflection mechanisms
package reflection;
import java.lang.reflect.Method;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;
/**
* Generics are obtained through reflection mechanisms
* @author Administrator
*
*/
public class Test03 {
public static void main(String[] args) {
Class<?> c = Test03.class;
try {
System.out.println("******* Gets the type of the parameter value **********");
Method m1 = c.getDeclaredMethod("method01", Map.class,List.class);
Type[] types = m1.getGenericParameterTypes();
for(Type t:types){
System.out.println(t.getTypeName());
System.out.println(t.toString());
}
System.out.println("******* Gets the type of the return value **********");
Method m2 = c.getDeclaredMethod("method02");
Type ret = m2.getGenericReturnType();
System.out.println(ret.getTypeName());
System.out.println(ret.toString());
} catch (NoSuchMethodException | SecurityException e) {
e.printStackTrace();
}
}
public void method01(Map<String,String> args1,List<Integer> args2){
}
public Map<String,String> method02(){
return null;
}
}
Get generic information through reflection mechanisms
output:
java.util.Map<java.lang.String, java.lang.String>
java.util.Map<java.lang.String, java.lang.String>
java.util.Map<java.lang.String, java.lang.String>
java.util.Map<java.lang.String, java.lang.String>
java.util.List<java.lang.Integer>
java.util.List<java.lang.Integer>
View Console