[JAVA] What I tried when I wanted to get all the fields of a bean

Keep a reminder of the process of trial and error to get all the fields in the bean.

Thing you want to do

I want to get all the fields in the bean with Iterator. ** "Field1, Field2, Field3, Field4" ** can be acquired for the following beans.

Fields.java

public class Fields {
  
  //field
  private String Field1;
  private String Field2;
  private String Field3;
  private String Field4;
  //constructor
  Fields(String Field1, String Field2, String Field3, String Field4) {
    this.Field1 = Field1;
    this.Field2 = Field2;
    this.Field3 = Field3;
    this.Field4 = Field4;
  }
  // settet,getter omitted
  // .
  // .
  // .
}

What I tried 1

Obtained using ** PropertyUtils / BeanUtils class ** of ** "Apache Commons" ** which is an external library of Java

** PropertyUtils ** and ** BeanUtils ** are classes that handle ** JavaBeans **. [^ 1] Among them, using the describe (Object bean) method that converts each value in the bean to Map and returns it, ** Convert Bean to Map → Get key value of converted Map (field name) ** I decided to get all the fields in the bean.

[^ 1]: Click here for a reference site on how to use BeanUtils. Java BeanUtils memo (Hishidama's commons-BeanUtils Memo)

Method

Fields.java(Use PropertyUtils)

 // PropertyUtils#Get all fields using describe
 public Iterator<?> getNames() 
		  throws IllegalAccessException, InvocationTargetException, NoSuchMethodException{
    try{
        //Convert Bean to Map → Get key value of converted Map (field name)
        return PropertyUtils.describe(this).keySet().iterator();
    }catch(IllegalAccessException e){
    	throw e;
    }catch(InvocationTargetException e){
    	throw e;
    }catch(NoSuchMethodException e){
    	throw e;
    }
  }

Fields.java(Use BeanUtils)

  // BeanUtilsUtils#Get all fields using describe
  public Iterator<?> getNames1() 
		  throws IllegalAccessException, InvocationTargetException, NoSuchMethodException{
	    try{
            //Convert Bean to Map → Get key value of converted Map (field name)
	        return BeanUtils.describe(this).keySet().iterator();
	    }catch(IllegalAccessException e){
	    	throw e;
	    }catch(InvocationTargetException e){
	    	throw e;
	    }catch(NoSuchMethodException e){
	    	throw e;
	    }
	  }

Execution class

FieldGet.java

package test;

import java.lang.reflect.InvocationTargetException;
import java.util.Iterator;

public class FieldGet {
  public static void main (String args[]){

    Fields field = new Fields("Field 1","Field 2","Field 3","Field 4");
    Iterator<?> propertyUtils;
    Iterator<?> beanUtils;
	try {
		// PropertyUtils#Get all fields using describe
		propertyUtils = field.getNames();
		// BeanUtilsUtils#Get all fields using describe
		beanUtils = field.getNames1();
		System.out.println("--Use PropertyUtils--");
	    while(propertyUtils.hasNext()){
	        System.out.println(propertyUtils.next());
	    }
		System.out.println("--Using BeanUtils--");
	    while(beanUtils.hasNext()){
	        System.out.println(beanUtils.next());
	    }
	} catch (IllegalAccessException e) {
		e.printStackTrace();
	} catch (InvocationTargetException e) {
		e.printStackTrace();
	} catch (NoSuchMethodException e) {
		e.printStackTrace();
    }
  }
}

Execution result

Execution result

--Use PropertyUtils--
Field1
Field3
class
Field2
Field4
--Using BeanUtils--
Field1
Field3
class
Field2
Field4

I was able to get the field name of the bean, There seems to be room for improvement, such as getting a "class" that is not a field name, or not in ascending order.

Also, while running it several times ... ʻInvocationTargetException occurred at PropertyUtils / BeanUtils.describe (this) `, and the field could not be retrieved. I don't know the exact cause, so I decided to consider getting the field name in another way.

Execution result(part)

Caused by: java.lang.reflect.InvocationTargetException
	... 1024 more
Caused by: java.lang.StackOverflowError

What I tried 2

Use reflection

Bean fields can also be obtained ** with the reflection API **. [^ 2] With getDeclaredFields (), ** all access modifier (public, protected, default, private) fields can be retrieved as an array of Field class **.

[^ 2]: Get field with Java reflection API

Method

Fields.java(Use reflection)

  // Class#Get all fields using getDeclaredFields
  public Iterator<?> getNames2(){
        //Get fields for all access modifiers
    	Field[] tmpField = this.getClass().getDeclaredFields();
        //Generate a list to return with Iterator
    	List<String> Field = new ArrayList<String>();
    	for(int i = 0; i < tmpField.length; i++) {
            // getName()Get the field with
    		Field.add(tmpField[i].getName());
    	}
    	return Field.iterator();
    }

Execution class

FieldGet.java

package test;

import java.util.Iterator;

public class FieldGet {
  public static void main (String args[]){

    Fields field = new Fields("Field 1","Field 2","Field 3","Field 4");
    terator<?> reflection;
    // Class#Get all fields using getDeclaredFields
    reflection = field.getNames2();
    System.out.println("--Use reflection--");
    while(propertyUtils.hasNext()){
        System.out.println(propertyUtils.next());
    }
  }
}

Execution result

Execution result

--Use reflection--
Field1
Field2
Field3
Field4

Now, as expected, we were able to get all the fields.

Extra edition

Get all fields in Struts DynaActionForm

You can also get all fields in Struts DynaActionForm. ** ✳︎ The idea is the same as "What I tried 1" **

Step 1: Create a class that inherits DynaValidatorForm or DynaActionForm Step 2: Implement the method with the same idea ** as "What I tried 1" ** (Convert form to Map → Get key value of converted Map (field name)) ** 　 Struts DynaActionForm has a getMap () method that converts the property defined in DynaActionForm to Map type and returns it **, so if you use that method, it is the same as "What I saw 1" It is possible to get all fields with the idea of. Is it something like ** equivalent to the describe (Object bean) method of the PropertyUtils / BeanUtils class **? I'm not sure about the detailed internal processing ...

StrutsDynaValidatorForm.java

package test;

import java.util.Iterator;
import org.apache.struts.validator.DynaValidatorForm;

public class StrutsDynaValidatorForm extends DynaValidatorForm {
	public Iterator getNames() {		
		return this.getMap().keySet().iterator();		
	}
}