Fill bean data from HashMap using Reflection

[Ravi Sree]

Hi all,

I was trying on a small example on Reflection by which i had to fill the values in a bean from a HashMap.
This is the code as it goes: -

TempBean.java

package net;

import java.io.Serializable;

public class TempBean implements Serializable
{
	private String name;
	private String address;

	public TempBean()
	{
		this.name = null;
		this.address = null;
	}

	public void setName(String _name)
	{ this.name = _name; }

	public String getName()
	{ return this.name; }	

	public void setAddress(String _address)
	{ this.address = _address; }

	public String getAddress()
	{ return this.address; }

	public void printVals()
	{
		System.out.println("-------printing from bean---------");
		System.out.println("name = " + getName());
		System.out.println("address = " + getAddress());
	}
}

ReflectUtil.java

package net;

import java.beans.PropertyDescriptor;
import java.beans.IntrospectionException;
import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;

public class ReflectUtil
{
	private ReflectUtil()
	{}

	public static Method getReadMethod(PropertyDescriptor pDescriptor) throws IntrospectionException, NoSuchMethodException
	{
		Method readMethod = null;
		readMethod = pDescriptor.getReadMethod();
		return readMethod;
	}

	public static Method getWriteMethod(PropertyDescriptor pDescriptor) throws IntrospectionException, NoSuchMethodException
	{
		Method writeMethod = null;
		writeMethod = pDescriptor.getWriteMethod();
		return writeMethod;
	}

	public static Object invokeMethod(Method method, Object bean, Object value) throws Exception
	{
		try
		{
			return method.invoke(bean, value);
		}
		catch(InvocationTargetException itex)
		{
			throw new Exception(itex.getMessage());
		}
		catch(IllegalArgumentException iaex)
		{
			throw new Exception(iaex.getMessage());
		}
		catch(IllegalAccessException iaex)
		{
			throw new Exception(iaex.getMessage());
		}
	}
}

ReflectionTester.java

package net;

import java.util.HashMap;
import java.beans.BeanInfo;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.beans.IntrospectionException;
import java.lang.reflect.Method;
import net.TempBean;

public class ReflectionTester
{
	private static TempBean tempBean;
	private static Class myClass;

	public ReflectionTester()
	{
		this.tempBean = null;
		this.myClass = null;
	}

	private static TempBean setValues(HashMap values)
	{
		Method method = null;
		TempBean tempBean2 = new TempBean();
		myClass = tempBean2.getClass();
		try
		{
			BeanInfo beanInfo = Introspector.getBeanInfo(myClass);
			PropertyDescriptor[] pDescriptors = beanInfo.getPropertyDescriptors();
			PropertyDescriptor[] descriptors = new PropertyDescriptor[pDescriptors.length];
			int j=0;
			for(int i=0; i<pDescriptors.length; i++)
			{
				System.out.println("pDescriptors[" + i + "] = " + pDescriptors[j].getName());
				if(pDescriptors[i].getName().equals("class"))
				{
					System.out.println("pDescriptors[" + i +"].getName().equals('class') condition true");				
//					j=1;
				}
				else
				{
					System.out.println("j = " + j);
					descriptors[i] = pDescriptors[i];
				}
				j++;
			}
			System.out.println("decriptors[].length = " + descriptors.length);
			for(int k=0; k<values.size(); k++)
			{
				String propName = descriptors[k].getName();                   ///////// This where i get a NullPointerException
				String value = (String) values.get(propName);	
				method = ReflectUtil.getWriteMethod(descriptors[k]);
				ReflectUtil.invokeMethod(method, tempBean2, value);
			}
		}
		catch(NoSuchMethodException nsaex)
		{
			System.out.println("Error1: - " + nsaex.getMessage());
		}
		catch(IntrospectionException ex)
		{
			System.out.println("Error2: - " + ex.getMessage());
		}
		catch(Exception ex)
		{
			System.out.println("Error3: -" + ex.getMessage());
			ex.printStackTrace();
		}
		return tempBean2;
	}

	public static void main(String[] args)
	{
		TempBean myTempBean = new TempBean();
		HashMap map = new HashMap();
		map.put("name", args[0]);
		map.put("address", args[1]);
		myTempBean = setValues(map);
		myTempBean.printVals(); 
	}
}

But on executing ReflectionTester with the arguments, A NullPointerException is getting thrown.
Please suggest where am i going wrong here.

Thanks in advance.
Sree

[Rob Spoor]

If pDescriptors[i].getName().equals("class") you will get a "gap" in your descriptors array; the value will remain null at that point. You will later encounter this null value, and when you try to get its name the exception is thrown. Change descriptors from a PropertyDescriptor[] to a List<PropertyDescriptor> to make sure you won't get any null references in it. Note that you'll need to replace descriptors[i] = pDescriptors[i] with descriptors.add(pDescriptors[i]), descriptors.length with descriptors.size() and descriptors[k] with descriptors.get(k).

That leaves another odd issue. You loop over the number of values given, which can definitely be different from the number of properties. Change the for loop:

for (int k = 0; k < descriptors.size(); k++)

The final question: what will you do if a property has no entry in the map?


Oh, and your ReflectionUtil class is superfluous. All it does is forward the call to its arguments. ReflectionUtil.getReadMethod(descriptors[k]) is the same as descriptors[k].getReadMethod(), and ReflectionUtil.invokeMethod(method, tempBean2, value) is the same as method.invoke(tempBean2, value). You won't even need to catch the NoSuchMethodException anymore.

[Wouter Oet]

There are a couple of things weird about your code:

Method readMethod = null;   
readMethod = pDescriptor.getReadMethod();   
return readMethod;    
// This method has a lot of unnecessary code and it does really
// nothing that can't be done in the code that calls this method.  

public static Object invokeMethod(Method method, Object bean, Object value) throws Exception   
    {   
        try   
        {   
            return method.invoke(bean, value);   
        }   
        catch(InvocationTargetException itex)   
        {   
            throw new Exception(itex.getMessage());   
        }   
        catch(IllegalArgumentException iaex)   
        {   
            throw new Exception(iaex.getMessage());   
        }   
        catch(IllegalAccessException iaex)   
        {   
            throw new Exception(iaex.getMessage());   
        }   
    }   // Why wrap all the specific Exception is a general Exception? You will lose all your stacktrace info.    

You're not using generics. And probably your problem is that you are iterating over the HashMap size and not over the
descriptors size. And why do you copy all the values into a new array except for the class entries? Why not just skip them?

[Campbell Ritchie]

Reflection is hardly a "beginning" topic. Moving thread.

[Ravi Sree]

Hi again,

Thanks for all your comments.

@Rob Prime-
I followed your suggestions, made the changes in the method. and it gave me what i wanted, Thanks friend

ReflectionTester.java

	private static TempBean setValues(HashMap values)
	{
		Method method = null;
		TempBean tempBean2 = new TempBean();
		myClass = tempBean2.getClass();
		try
		{
			BeanInfo beanInfo = Introspector.getBeanInfo(myClass);
			PropertyDescriptor[] pDescriptors = beanInfo.getPropertyDescriptors();

	/*		PropertyDescriptor[] descriptors = new PropertyDescriptor[pDescriptors.length];
			int j=0;
			for(int i=0; i<pDescriptors.length; i++)
			{
				System.out.println("pDescriptors[" + i + "] = " + pDescriptors[j].getName());
				if(pDescriptors[i].getName().equals("class"))
				{
					System.out.println("pDescriptors[" + i +"].getName().equals('class') condition true");				
//					j=1;
				}
				else
				{
					System.out.println("j = " + j);
					descriptors[i] = pDescriptors[j];
				}
				j++;
			}
			System.out.println("decriptors[].length = " + descriptors.length);
			for(int k=0; k<values.size(); k++)
			{
				String propName = descriptors[k].getName();
				String value = (String) values.get(propName);	
				method = ReflectUtil.getWriteMethod(descriptors[k]);
				ReflectUtil.invokeMethod(method, tempBean2, value);
			}
	*/
			List<PropertyDescriptor> descriptors = new ArrayList<PropertyDescriptor>();
			for(int i=0; i<pDescriptors.length; i++)
			{
				System.out.println("pDescriptors[" + i + "] = " + pDescriptors[i].getName());
				if(!pDescriptors[i].getName().equals("class"))
				{
					descriptors.add(pDescriptors[i]);
				}
			}
			for(int j=0; j><descriptors.size(); j++)
			{
				String propName = descriptors.get(j).getName();
				String value = (String) values.get(propName);	
				method = ReflectUtil.getWriteMethod(descriptors.get(j));
				ReflectUtil.invokeMethod(method, tempBean2, value);				
			}
		}
		catch(NoSuchMethodException nsaex)
		{
			System.out.println("Error1: - " + nsaex.getMessage());
		}
		catch(IntrospectionException ex)
		{
			System.out.println("Error2: - " + ex.getMessage());
		}
		catch(Exception ex)
		{
			System.out.println("Error3: -" + ex.getMessage());
			ex.printStackTrace();
		}
		return tempBean2;
	}

@Wouter Oet
I will make sure i remove the unnecessary code as you said.
Appreciate your help.

The code after undergoing the changes as Rob had suggested, is working without an error.

>

[Wouter Oet]

You could improve your code by merging the two for-loops into one.

[Ravi Sree]

Wouter Oet wrote:There are a couple of things weird about your code:

Method readMethod = null;   
readMethod = pDescriptor.getReadMethod();   
return readMethod;    
// This method has a lot of unnecessary code and it does really
// nothing that can't be done in the code that calls this method.  

public static Object invokeMethod(Method method, Object bean, Object value) throws Exception   
    {   
        try   
        {   
            return method.invoke(bean, value);   
        }   
        catch(InvocationTargetException itex)   
        {   
            throw new Exception(itex.getMessage());   
        }   
        catch(IllegalArgumentException iaex)   
        {   
            throw new Exception(iaex.getMessage());   
        }   
        catch(IllegalAccessException iaex)   
        {   
            throw new Exception(iaex.getMessage());   
        }   
    }   // Why wrap all the specific Exception is a general Exception? You will lose all your stacktrace info.    

You're not using generics. And probably your problem is that you are iterating over the HashMap size and not over the
descriptors size. And why do you copy all the values into a new array except for the class entries? Why not just skip them?

Hi,

I think i have removed unnecessary code as suggested earlier.
But I again ran into another problem similiar like this..
These are the classes : -

FormUtil

package net.utils;

import java.util.HashMap;
import java.util.Enumeration;

import javax.servlet.http.HttpServletRequest;
import net.utils.BeanUtil;

public class FormUtil
{
	private static final String PACKAGE = "net.forms.";

	private FormUtil()
	{}

        public static HashMap<String, String> getParameterMap(HttpServletRequest req)
	{
		HashMap<String, String> paramMap = new HashMap<String, String>();
		Enumeration<?> parameterNames = req.getParameterNames();
		while(parameterNames.hasMoreElements())
		{
			String key = (String) parameterNames.nextElement();			
			String value = getFieldValue(req, key);
			paramMap.put(key, value);
		}
		return paramMap;
	}

	private static String getFieldValue(HttpServletRequest req, String fieldName)
	{
		String fieldValue = null;
		fieldValue = req.getParameter(fieldName);
		return fieldValue;
	}

	@SuppressWarnings("unchecked")
	public static void parseRequest(HttpServletRequest req)
	{
		HashMap formVals = getParameterMap(req);
		String formBeanName = (String) formVals.get("formname");
		Class<?> clazzObj = getClass(formBeanName);
		try
		{
			BeanUtil.setFromMap(clazzObj, formVals);
			req.setAttribute("form", clazzObj);
		}
		catch(Exception ex)
		{
			System.out.println("Error: - " + ex.getMessage());
			ex.printStackTrace();
		}
	}

	private static Class<?> getClass(String formbeanName)
	{
		Class<?> clazzObj = null;
		try
		{
			clazzObj = Class.forName(PACKAGE + formbeanName);
		}
		catch(ClassNotFoundException cnfe)
		{
			System.out.println("Error: - " + cnfe.getMessage());
		}
		catch(Exception ex)
		{
			System.out.println("Error: - " + ex.getMessage());
		}
		return clazzObj;
	}
}

BaseForm

package net.forms;

import java.util.Map;
import java.util.HashMap;

import java.io.Serializable;

public abstract class BaseForm implements Serializable
{
	protected HashMap errors = new HashMap();
	protected HashMap messages = new HashMap();
	
	public void addError(String key, String _error)
	{
		errors.put(key, _error);
	}

	public String getError(String key)
	{
		return (String) errors.get(key);
	}

	public void addMessage(String key, String _message)
	{
		messages.put(key, _message);
	}

	public String getMessage(String key)
	{
		return (String) messages.get(key);
	}	
}

LoginForm

package net.forms;

import net.forms.BaseForm;

public class LoginForm extends BaseForm
{
	private String loginId;
	private String password;

	public LoginForm()
	{
		this.loginId = null;
		this.password = null;
	}

	public void setLoginId(String _loginId)
	{ this.loginId = _loginId; }

	public String getLoginId()
	{ return this.loginId; }

	public void setPassword(String _password)
	{ this.password = _password; }

	public String getPassword()
	{ return this.password; }

}

BeanUtil

package net.utils;

import java.util.HashMap;

import java.beans.BeanInfo;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.beans.IntrospectionException;

import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;

public class BeanUtil
{
     public static void setFromMap(Class clazz, HashMap<String, String> propValues) throws Exception
	{
		PropertyDescriptor[] pdescriptors = null;
		try
		{
			BeanInfo beanInfo = Introspector.getBeanInfo(clazz);
			pdescriptors = beanInfo.getPropertyDescriptors();
			for(int i=0; i<pdescriptors.length; i++)
			{
				String descriptorName = pdescriptors[i].getName();
				if(!(descriptorName.equals("class")))
				{
					System.out.println("PropertyDescriptor [" + i + "] = " + descriptorName);
					String propName = descriptorName;
					String value = (String) propValues.get(propName);
					if(value != null)
					{
						Object[] objArray = new Object[1];
						objArray[0] = value;
						Method writeMethod = pdescriptors[i].getWriteMethod();
						writeMethod.invoke(clazz, objArray);                 //I am getting IllegalArgumentException here
					}
				}
			}
		}
		catch(IntrospectionException iex)
		{
//			throw iex;
			iex.printStackTrace();
		}
		catch(InvocationTargetException itex)
		{
//			throw itex;
			itex.printStackTrace();
		}
		catch(IllegalArgumentException iaex)
		{
//			throw iaex;
			iaex.printStackTrace();
		}
		catch(IllegalAccessException ilaex)
		{
//			throw ilaex;
			ilaex.printStackTrace();
		}
}

Here i am trying the a similiar example to set values from hashmap to the class,
but i am getting a error(IllegalArgugmentException) while invoking the method,
Please suggest what can be the reason and what to do to correct this...

Thanks in advance.

Regards,
Sree

[Paul Clapham]

The reason? When you called the invoke() method, an error was thrown. Perhaps by the method you were invoking? You don't know because you didn't...

What can be done about it?
Look at the stack trace to see what the error was. Not only will there be a stack trace for the InvocationTargetException, there will also be a stack trace for the actual exception which was thrown by the invoked method.

[Ravi Sree]

Paul Clapham wrote:The reason? When you called the invoke() method, an error was thrown. Perhaps by the method you were invoking? You don't know because you didn't...

What can be done about it?
Look at the stack trace to see what the error was. Not only will there be a stack trace for the InvocationTargetException, there will also be a stack trace for the actual exception which was thrown by the invoked method.

Hi Paul,

Thanks for replying,
I had written InvocationTargetException for IllegalArgumentException by mistake,
the stacktrace shown is follows:-

java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at net.utils.BeanUtil.setFromMap(BeanUtil.java:56)
at net.utils.FormUtil.parseRequest(FormUtil.java:44)

I still cant make out what made it to throw this exception.
Please forgive my typeing-mistake and suggest what should be done to correct this.

Regards,
Sree

[Rob Spoor]

Your first argument is a Class object, but it needs to be an instance of the class represented by the Class object. For example, if clazz == String.class you would need to pass a String.

[Ravi Sree]

Rob Prime wrote:
Your first argument is a Class object, but it needs to be an instance of the class represented by the Class object. For example, if clazz == String.class you would need to pass a String.

Hi Rob,

I did the change

...
writeMethod.invoke(clazz.newInstance(), objArray);

Please suggest if this ok as this working right as i needed.

Regards,
Sree

[Rob Spoor]

It depends. If it's ok to call the method on a new instance that isn't referred to from anywhere else, then that code should work. If you need to call the method on an existing instance then you'll need to pass that instance to the method. Something tells me that's the case.

I'd change your method signature slightly:

public static void setFromMap(Object object, HashMap<String, String> propValues) throws Exception
{
    Class<?> clazz = object.getClass();
    ...
    writeMethod.invoke(object, objArray);
    ...
}

You will now be setting the values to an existing object.

[Ravi Sree]

Rob Prime wrote:It depends. If it's ok to call the method on a new instance that isn't referred to from anywhere else, then that code should work. If you need to call the method on an existing instance then you'll need to pass that instance to the method. Something tells me that's the case.

I'd change your method signature slightly:

public static void setFromMap(Object object, HashMap<String, String> propValues) throws Exception
{
    Class<?> clazz = object.getClass();
    ...
    writeMethod.invoke(object, objArray);
    ...
}

You will now be setting the values to an existing object.

I did the changes as you said,
if i pass the argument as object i get the class after getClass() as of type <java.lang.Class>
and not of the type <net.forms.LoginForm>, and i plan to set this instance in request attribute

I get a hint that i will have to use java Generics here, but am not much aquainted with it.

Please guide about what can be done to make method work right.

Sree

[Rob Spoor]

You no longer should pass the Class object as your first argument but an instance of that class - the instance whose fields you want to set.

[Ravi Sree]

Rob Prime wrote:You no longer should pass the Class object as your first argument but an instance of that class - the instance whose fields you want to set.

Could you elaborate with a code sample please.

Sree

[Rob Spoor]

TestBean testBean = new TestBean();
HashMap<String,String> values = < however you get it now >;
BeanUtil.setFromMap(testBean, values); // tadaaaa. testBean will now have its values set

[Ravi Sree]

Rob Prime wrote:

TestBean testBean = new TestBean();
HashMap<String,String> values = < however you get it now >;
BeanUtil.setFromMap(testBean, values); // tadaaaa. testBean will now have its values set

Hi ,

I tried to improve the code a bit more,
Even though i have a general idea how the
code should work, I am havent yet appplied
this method successfully.
I think using Java Generics will work here,
so i started with these changes, but couldn't
go any further.

BeanUtil

	public static <T extends Object> T setFromMap(T bean, HashMap<String, String> propValues) throws Exception
	{
		PropertyDescriptor[] pdescriptors = null;
		try
		{
			BeanInfo beanInfo = Introspector.getBeanInfo(bean.getClass());
			pdescriptors = beanInfo.getPropertyDescriptors();
			for(int i=0; i<pdescriptors.length; i++)
			{
				String descriptorName = pdescriptors[i].getName();
				if(!(descriptorName.equals("class")))
				{
					System.out.println("PropertyDescriptor [" + i + "] = " + descriptorName);
					String propName = descriptorName;
					String value = (String) propValues.get(propName);
					if(value != null)
					{
						Object[] objArray = new Object[1];
						objArray[0] = value;
						Method writeMethod = pdescriptors[i].getWriteMethod();
						writeMethod.invoke((T)new Object(), objArray);
					}
				}
			}
		}
		catch(IntrospectionException iex)
		{
//			throw iex;
			iex.printStackTrace();
		}
		catch(InvocationTargetException itex)
		{
//			throw itex;
			itex.printStackTrace();
		}
		catch(IllegalArgumentException iaex)
		{
//			throw iaex;
			iaex.printStackTrace();
		}
		catch(IllegalAccessException ilaex)
		{
//			throw ilaex;
			ilaex.printStackTrace();
		}
		/*
		 catch(InstantiationException iex)
		{
			throw iex;
		}
		catch(NoSuchMethodException nsmex)
		{
			throw nsmex;
		}
		*/
		return bean;
	}

I plan to send the object to the method,
set the values and return the same object
back, where (object extends BaseForm)

I am missing the core concept of Java Generics here,
but want to clear them too.

Please guide me.

Thanks in advance,
Sree">

[Rob Spoor]

Why do you create a new Object to pass into writeMethod.invoke? Why not use the argument to the method, as I've shown before?

[Ravi Sree]

Rob Prime wrote:Why do you create a new Object to pass into writeMethod.invoke? Why not use the argument to the method, as I've shown before?

quote=Rob Prime]Why do you create a new Object to pass into writeMethod.invoke? Why not use the argument to the method, as I've shown before?
Hi Rob,
Sorry for replying late.
I followed your suggestion and used it Java Generics
and was able to find the required solution.

              public static <T extends Object> T setFromMap(Class<T> beanClazz, HashMap<String, String> propValues) throws Exception
	{
		T bean = (T) beanClazz.newInstance();
		Object obj = new Object();
		PropertyDescriptor[] pdescriptors = null;
		try
		{
			BeanInfo beanInfo = Introspector.getBeanInfo(beanClazz);
			pdescriptors = beanInfo.getPropertyDescriptors();
			for(int i=0; i<pdescriptors.length; i++)
			{
				String descriptorName = pdescriptors[i].getName();
				if(!(descriptorName.equals("class")))
				{
//					System.out.println("PropertyDescriptor [" + i + "] = " + descriptorName);
					String propName = descriptorName;
					String value = (String) propValues.get(propName);
					if(value != null)
					{
						Object[] objArray = new Object[1];
						objArray[0] = value;
						Method writeMethod = pdescriptors[i].getWriteMethod();
						writeMethod.invoke(bean, objArray);
					}
				}
			}
		}
		catch(IntrospectionException iex)
		{
			throw iex;
		}
		catch(InvocationTargetException itex)
		{
			throw itex;
		}
		catch(IllegalArgumentException iaex)
		{
			throw iaex;
		}
		catch(IllegalAccessException ilaex)
		{
			throw ilaex;
		}
		/*
		 catch(InstantiationException iex)
		{
			throw iex;
		}
		catch(NoSuchMethodException nsmex)
		{
			throw nsmex;
		}
		*/
		return bean;
	}

Thanks,
Regards,
Sree">

[Rob Spoor]

I see two possible improvements:
- use Map<String,String> instead of HashMap<String,String> in the method signature to allow TreeMap as well.
- check if the method's parameter list expects only one String or Object:

Class<?>[] params = writeMethod.getParameterTypes();
if (params.length == 1 && params[0].isAssignableFrom(String.class)) // also allows Serializable etc
{
    Object[] objArray = new Object[1];   
    objArray[0] = value;
    writeMethod.invoke(bean, objArray);
}

This will prevent issues where the property is not a String property but for instance an Integer.

[Ravi Sree]

Rob Prime wrote:I see two possible improvements:
- use Map<String,String> instead of HashMap<String,String> in the method signature to allow TreeMap as well.
- check if the method's parameter list expects only one String or Object:

Class<?>[] params = writeMethod.getParameterTypes();
if (params.length == 1 && params[0].isAssignableFrom(String.class)) // also allows Serializable etc
{
    Object[] objArray = new Object[1];   
    objArray[0] = value;
    writeMethod.invoke(bean, objArray);
}

This will prevent issues where the property is not a String property but for instance an Integer.

Hi Rob,

Sorry to reply so late,

I have the made changes to my code as you suggested.

Thanks ,
Sree