How to get fields in an object through reflection?

Question

I have an object (mostly VO) in Java, and I don't know its type.
I need to get values ​​that are not null in this object.

How can I do that?

Answer 1

You can use Class#getDeclaredFields() to get all declared fields of this class. You can use Field#get() to get the value.

In short:

 Object someObject = getItSomehow(); for (Field field : someObject.getClass().getDeclaredFields()) { field.setAccessible(true); // You might want to set modifier to public first. Object value = field.get(someObject); if (value != null) { System.out.println(field.getName() + "=" + value); } } 

To learn more about reflection, check out Sun's tutorial on this .

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However, fields do not necessarily all represent VO properties. You prefer to define public methods, starting with get or is , and then calling it to capture the values ​​of real properties.

 for (Method method : someObject.getClass().getDeclaredMethods()) { if (Modifier.isPublic(method.getModifiers()) && method.getParameterTypes().length == 0 && method.getReturnType() != void.class && (method.getName().startsWith("get") || method.getName().startsWith("is")) ) { Object value = method.invoke(someObject); if (value != null) { System.out.println(method.getName() + "=" + value); } } } 

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This, in turn, says that there may be more elegant ways to solve your real problem. If you talk a little more about the functional requirement, for which, in your opinion, this is the right solution, we can offer the right solution. Many tools are available to massage the Javabei.

Answer 2

Here is a quick and dirty method that does what you want in general. You will need to add exception handling, and you probably want to cache BeanInfo types in weakhashmap.

 public Map<String, Object> getNonNullProperties(final Object thingy) { final Map<String, Object> nonNullProperties = new TreeMap<String, Object>(); try { final BeanInfo beanInfo = Introspector.getBeanInfo(thingy .getClass()); for (final PropertyDescriptor descriptor : beanInfo .getPropertyDescriptors()) { try { final Object propertyValue = descriptor.getReadMethod() .invoke(thingy); if (propertyValue != null) { nonNullProperties.put(descriptor.getName(), propertyValue); } } catch (final IllegalArgumentException e) { // handle this please } catch (final IllegalAccessException e) { // and this also } catch (final InvocationTargetException e) { // and this, too } } } catch (final IntrospectionException e) { // do something sensible here } return nonNullProperties; } 

See the following links:

• BeanInfo (JavaDoc)
• Introspector.getBeanInfo (class) (JavaDoc)
• Introspection (Sun Java Tutorial)

Answer 3

I have an object (mostly VO) in Java and I don't know its type. I need to get values ​​that are not null in this object.

Perhaps you don't need to speculate for this - here is a simple OO design that can solve your problem:

• Add a Validation interface that issues a validate method that validates the fields and returns whatever is appropriate.
• Implement an interface and method for all VOs.
• When you get a VO, even if that particular type is unknown, you can cast it to Validation and easily check.

I assume that you need a field that is null to display the error message in a general way, so that should be enough. Let me know if for some reason this does not work for you.