Thread: ObjectMapper class: readValue method fails

I'm putting together a demo website using JSP/JQuery on the client side and java on the server side. I am calling Jquery's .ajax() method to pass data to a servlet, trying to pass JSON data from the client to the server.

I found this example and modeled my code on it:

https://hmkcode.com/java-servlet-sen...g-jquery-ajax/

Rather that using the 'Article' class mentioned at that URL, I used my own example class 'EmailRecord' shown at the end of this post. I have tested this class and it passes my test.

My copy of the example java code looks like the code shown below, except I have, for brevity, eliminated my debug statements and sanity checking. In my actual code, I test that my objects are not null, so I know that values for 'br' and 'mapper' are valid. In my test code, I print out the value of the variable 'json', which is:

returnVal=JJA20&email=j%40j.c

I also call 'mapper.copy()', also not shown in the code below, to verify that my 'mapper' variable is indeed a valid handle to an ObjectMapper instance.

    // 1. get received JSON data from request
    String json = ""; 
    BufferedReader br = 
        new BufferedReader(new InputStreamReader(request.getInputStream()));
    json = br.readLine();
 
    // 2. initiate jackson mapper
    ObjectMapper mapper = new ObjectMapper();
 
    // 3. Convert received JSON to EmailRecord
    EmailRecord er = mapper.readValue(json, EmailRecord.class);
 
    // 4. Set response type to JSON
    response.setContentType("application/json");
 
    // 5. Add article to List<Article> (not needed in my case)
 
    // 6. Send List<Article> as JSON to client
    mapper.writeValue(response.getOutputStream(), er);
 
    return;

When I compile this code and run it, if fails at the line:

     EmailRecord er = mapper.readValue(json, EmailRecord.class);

After failling, I tried wrapping the line in a try/catch operation. It did indeed throws an exception, but so far I have not figured out how to get the stacktrace. The most likely reason I can think for the failure is that the servelet is not finding the 2nd argument, 'EmailRecord.class'. This class can be found at localhost:8080/bip/WEB-INF/classes/bfs/bip/Element.class, where 'bip' is my test website. I assume the mapper knows to look there, but this is only an assumption. If not true, that explains the failure.

So here are my questions:

1) Where does ObjectMapper readValue look for EmailRecord.class?

2) Is there a way to get a stacktrace from the exception that this thrown? I tried the following

        try {
            er = mapper.readValue(json, EmailRecord.class);
        }   
        catch ( Exception e ) { 
            e.printStackTrace();
        }

but I don't know where the stacktrace will be directed.

3) For what other reasons would the readValue method fail?

BTW, the reason I am not attaching any source code is that when I push the "Manage Attachments" button on the web page, nothing happens. I tried it in both Firefox and Opera and got the same results.

Below is the EmailRecord.java class that I previously mentioned.

package bfs.bip;
 
import java.io.*;
 
public class EmailRecord
{
    private String d_addr;
    private String d_returnVal;
 
public EmailRecord()
{
    setAddr(""); 
    setReturnVal("");
}
 
public void setAddr(String addr)
{
    d_addr = addr;
}
 
public String getAddr()
{
    return(d_addr);
}
public void setReturnVal(String val)
{
    d_returnVal = val;
}
 
public String getReturnVal()
{
    return(d_returnVal);
}
 
public static void main(String args[])
{
 
    EmailRecord er = null;
    er = new EmailRecord();
 
    er.setReturnVal("Looks Good");
    System.out.println("Return Value = " + er.getReturnVal());
    er.setAddr("j@j.c");
    System.out.println("email = " + er.getAddr());
}
 
}

I don't know if anyone on this site is knowledgeable in JQuery, ajax or JSON.