Cannot set Boolean value through reflection

Question

I am not able to set a Boolean value to a field using Java reflection. The field data type is java.lang.Boolean. however, I am able to set the value if the data type is primitive type i.e boolean.

Here is a simple VO with Boolean type and primitive type:

public class TestVO {
    private Boolean bigBoolean;
    private boolean smallBoolean;
}

Here is my java reflection code:

public class TestClass {
    public static void main(String args[])
            throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException, SecurityException {
        TestVO testVO1 = new TestVO();

        Class testVO = testVO1.getClass();
        Field smallBooleanField = TestVO.class.getDeclaredField("smallBoolean");
        Field bigBooleanField = TestVO.class.getDeclaredField("bigBoolean");

        String name1 = smallBooleanField.getName();
        System.out.println("SmallBoolean Fieldname is: " + name1);

        smallBooleanField.setAccessible(true);

        // get the value of this private field
        Boolean fieldValue = (Boolean) smallBooleanField.get(testVO1);
        System.out.println("fieldValue = " + fieldValue);

        smallBooleanField.setAccessible(true);
        smallBooleanField.setBoolean(testVO1, true);

        // get the value of this private field
        fieldValue = (Boolean) smallBooleanField.get(testVO1);
        System.out.println("fieldValue = " + fieldValue);

        name1 = bigBooleanField.getName();
        System.out.println("bigBooleanField Fieldname is: " + name1);

        bigBooleanField.setAccessible(true);

        // get the value of this private field
        fieldValue = (Boolean) bigBooleanField.get(testVO1);
        System.out.println("fieldValue = " + fieldValue);

        bigBooleanField.setAccessible(true);
        bigBooleanField.setBoolean(testVO1, new Boolean(true));

        // get the value of this private field
        fieldValue = (Boolean) bigBooleanField.get(testVO1);
        System.out.println("fieldValue = " + fieldValue);

    }
}

Output is:

SmallBoolean Fieldname is: smallBoolean
fieldValue = false
fieldValue = true
bigBooleanField Fieldname is: bigBoolean
fieldValue = null
Exception in thread "main" java.lang.IllegalArgumentException: Can not set java.lang.Boolean field TestVO.bigBoolean to (boolean)true
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:175)
    at sun.reflect.UnsafeObjectFieldAccessorImpl.setBoolean(UnsafeObjectFieldAccessorImpl.java:90)
    at java.lang.reflect.Field.setBoolean(Field.java:795)
    at TestClass.main(TestClass.java:44)

---

I tried to set the bigBoolean value with new Boolean(true), Boolean.TRUE, true etc. nothing works.

Answer 1

According to this, bigBoolean.setBoolean() is invoked to set a field that is of the reference type Boolean with a value of primitive type. In the non-reflection equivalent Boolean val = true, the compiler would convert (or box) the primitive type 'true ' to a reference type as new Boolean(True) so that its type checking will accept the statement.

When using reflection, type checking only occurs at runtime so there is no opportunity to box the value. This forces to throw IllegalArgumentException due to Inconvertible Types