Get the class instance variables and print their values using reflection

Question

I have a 2 POJO classes with getters and setters,now i am trying to get all the class instance variables of that class.

I got to know that we can use reflection how to do it?

This is my POJO Class which will extend my reflection class.

class Details{

private int age;
private String name;

}

Reflection class is like this:

class Reflection{

public String toString(){

return all the fields of that class
}

Have you looked at *any* reflection tutorials or looked at any docs? — Dave Newton, Feb 27 '13 at 12:53
yes i have seen this http://stackoverflow.com/questions/744226/java-reflection-how-to-get-the-name-of-a-variable — Goofy, Feb 27 '13 at 12:54
If you want to create a new instance using reflection, refer this -> http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#newInstance%28%29 — Apurv, Feb 27 '13 at 12:57

Emiel · Accepted Answer · 2013-02-27T13:19:19.073

41

You could do something like this:

public void printFields(Object obj) throws Exception {
    Class<?> objClass = obj.getClass();

    Field[] fields = objClass.getFields();
    for(Field field : fields) {
        String name = field.getName();
        Object value = field.get(obj);

        System.out.println(name + ": " + value.toString());
    }
}

This would only print the public fields, to print private fields use class.getDeclaredFields recursively.

Or if you would extend the class:

public String toString() {
    try {
        StringBuffer sb = new StringBuffer();
        Class<?> objClass = this.getClass();

        Field[] fields = objClass.getFields();
        for(Field field : fields) {
            String name = field.getName();
            Object value = field.get(this);

            sb.append(name + ": " + value.toString() + "\n");
        }

        return sb.toString();
    } catch(Exception e) {
        e.printStackTrace();
        return null;
    }
}

edited Feb 27 '13 at 13:19

answered Feb 27 '13 at 12:58

Emiel

473
6
7

ok but can i override toString method and do it in the same way? which will return me all the fields? – Goofy Feb 27 '13 at 13:00
Sure, you would have to append the name/value to a string instead of printing it out and you would also need a reference to an instance of the Details class in your Reflection class. – Emiel Feb 27 '13 at 13:05
i wil tell you what i am trying to do if i extend the class which has toString method in it and print all the field names... but the prob is toString is a buit in method and cannot pass any parameters to that – Goofy Feb 27 '13 at 13:09
So, Details extends Reflection? If it does, substitute obj with this. – Emiel Feb 27 '13 at 13:10
@Mast3rPlan I couldn't properly understand this part: Object value = field.get(this); What is this supposed to be? What kind of Object? I tried many times to create a new object but I couldn't. Also, how could I access the value of a certain field and use it in a conditional statement? – Geosphere Jun 09 '16 at 22:03
mark the field as accessible using `field.setAccessible(true); ` before `Object value = field.get(obj);` – Arpit Aggarwal Aug 28 '16 at 07:02

score 3 · Answer 2 · answered Mar 15 '17 at 09:40

3

Adding one more line to the above code. If you want to access the private properties of the class use below line

field.setAccessible(true);

answered Mar 15 '17 at 09:40

Prasad

1,089
13
21

score 2 · Answer 3 · answered Jan 08 '17 at 20:09

    ClassLoader classLoader = Main.class.getClassLoader();
    try {
        Class cls = classLoader.loadClass("com.example.Example");
        Object clsObject = cls.newInstance();

        Field[] fields = cls.getFields();
        for (Field field : fields) {
            String name = field.getName();
            Object value = field.get(clsObject);

            System.out.println("Name : "+name+" Value : "+value);

        }
        System.out.println(cls.getName());


    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (InstantiationException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

score 1 · Answer 4 · edited Nov 08 '16 at 20:11

The resolution code or answer which mentioned above has one issue. To access the value of a private varible their access type must be set to true

Field[] fields = objClass.getDeclaredFields();
for (Field field : fields) {
    NotNull notNull = field.getAnnotation(NotNull.class);
    field.setAccessible(true);
}

else it will throw java.lang.IllegalAccessException. Class Reflection can not access a member of class Details with modifiers "private"

Get the class instance variables and print their values using reflection

4 Answers4

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