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Question

Shouldn't a method that receives java.lang.Object as input also receive javax.servlet.jsp.JspWriter as input?

Answer 1

A:

JspWriter is not a PrintWriter

you can wrap the original writer inside a print writer like this:

if (out instance of Writer) {
 PrintWriter p = new PrintWriter((Writer) out));
 p.print...
}

mohammad shamsi 2010-10-03 14:21:59

JspWriter can be cast to PrinterWriter though. If I add this little method, the problem will go away: public void print(JspWriter out) throws java.io.IOException { print (new PrintWriter(out)); }

Brian Kessler 2010-10-03 14:42:35

@brian kessler - the problem doesn't go away. If you change the IOException to an Exception, the problem is merely hidden. The comment you posted above will fail at runtime if you try it with a PrintWriter.

Tony Ennis 2010-10-03 15:07:09

The code above works fine with PrintWriter. Only JspWriter is failing, at least initially because Java isn't recognizing JspWriter as an Object (though I thought every class inherited from object?). If I add the immediately above method, both PrintWriter and JspWriter work ... but I'm trying to understand why Java doesn't seem to consider JspWriter an object and whether it would be possible to make the code above work withouth using another method to handle JspWriter.

Brian Kessler 2010-10-03 15:15:45

@Brian Kessler JspWriter and PrintWriter both are extending from Writer. how do you want cast them to each other?

mohammad shamsi 2010-10-03 15:20:32

@mohammad shamsi: I'm trying to see how far I can consolidate code which was identical for both types. As the code is otherwise identical, I'd like to catch both types with a single function and process them both using a single condition. Right now, catching them both with a single function seems the bigger headache.

Brian Kessler 2010-10-03 15:28:18

Answer 2

+1 A:

This is because the Java compiler tries to find the PrintWriter.print method on the JspWriter object. Although it has a print method, this method does not match because it is from another class. Java does not support duck-typing and goes to great lengths to prevent it.

It is also considered bad practice to use exceptions in programming logic.

You'll have to do sthg like

    try
    { 
       if (out instanceof PrintWriter) {
          ((PrintWriter) out).print(this.formattedString()); 
       } else if (out instanceof JspWriter) {
          ((JspWriter) out).print(this.formattedString()); 
       } else {
          throw new IllegalArgumentException("NameAndAddress.print expected ether a PrintWriter or a JspWriter but received a " + out.getClass().getName());
       }
    catch (Exception ex)
    { System.err.println("\"out\" is not a printable type: " + ex); }

BTW: methods in Java should start with a lowercase letter by convention.

Peter Tillemans 2010-10-03 14:22:18

Tried this, but Java still doesn't find the method to execute these conditions.

Brian Kessler 2010-10-03 14:39:01

I double checked : JspWriter.print(String s) and PrintWriter.print(String s) both exist. It should work. What is the error you're getting?

Peter Tillemans 2010-10-03 14:51:03

The error is, as above. The crux of it is: org.apache.jasper.JasperException: javax.servlet.ServletException: java.lang.NoSuchMethodError: hu.flux.helper.NameAndAddress.print(Ljavax/servlet/jsp/JspWriter;)V which, if I understand correctly, means that Java isn't recognizing JspWriter as an Object

Brian Kessler 2010-10-03 15:00:13

It could be that the object you're passing is loaded in a different classloader from the one the NameAndAddress class has been loaded. Is the NameAndAddress code loaded from a servlet or from the JspPage?

Peter Tillemans 2010-10-03 15:06:38

I'm calling the class from two places. I have a servlet calling with PrintWriter; this works fine. I have a Jsp page calling with JspWriter; this is the one that causes the problem if I don't have a method explicitly for JspWriter.

Brian Kessler 2010-10-03 15:18:29

I am at the end of my rope here, sorry. Good luck hunting the bug.

Peter Tillemans 2010-10-03 15:33:08

Answer 3

+3 A:

This exception says that your JSP code haven't been recompiled after you changed print(JspWriter) to print(Object), so it still tries to call print(JspWriter) and can't find it.

In order to force a recompilation you may modify your JSP page.

axtavt 2010-10-03 14:23:23

Right, either recompile or just move all that Java code out into a servlet where it belongs. Instead of modifying you can also choose to clean the work directory. Most IDE's offer a *Clean* context menu option in server entry.

BalusC 2010-10-03 14:32:24

Recompiled multiple times... that's not it, but cheers for the response.

Brian Kessler 2010-10-03 14:41:57

@Brian: Stacktrace remains the same after recompilation? If so, recompilation doesn't actually occur for some reason.

axtavt 2010-10-03 14:46:44

Not identical... Besides, I can make changes which will have the code functional (though not functioning as I'm trying to get it to function) and it works and then change things so that Java must realize that JspWriter inherits from either Object or Writer and it breaks again.

Brian Kessler 2010-10-03 14:58:34

something stagnant in your environment. do a clean build. restart your machine. format disk.

irreputable 2010-10-03 15:58:11

Answer 4

A:

Well you start your method by casting to a PrintWriter, so maybe the jvm optimized the method for you. Since JspWriter does not subclass from PrintWriter, your best bet is to write two methods:

public void print(JspWriter out) 
{ 
    if (out == null) return;
    try {
        out.print(this.FormattedString());
    } Except (IOException e) {
        // handle error 
    }
}

public void print(PrintWriter out) 
{ 
    if (out == null) return;
    try {
        out.print(this.FormattedString());
    } Except (IOException e) {
        // handle error 
    }
}

If only JspWriter and PrintWriter had a common interface... sigh.

Mike Axiak 2010-10-03 14:24:09

They both descend from Writer.

Tony Ennis 2010-10-03 14:28:01

JspWriter _can_ be cast to PrinterWriter though. If I add this little method, the problem will go away: public void print(JspWriter out) throws java.io.IOException { print (new PrintWriter(out)); }

Brian Kessler 2010-10-03 14:40:46

@brian kessler - Java will let us cast classes to other classes, but that doesn't mean that when it's time for that code to be executed that it will run. In the case of passing in an Object and casting whatever it _really_ is to a PrintWriter, it means we're simply turning off the error checking that the designers put in to the language.

Tony Ennis 2010-10-03 14:52:28

@Tony Ennis: Writer does not have a print() method, so using the writer interface wouldn't work.

Mike Axiak 2010-10-03 16:04:03

Answer 5

A:

Your print() casts Object to a PrintWriter. But when you pass in a JspWriter which isn't PrintWriter, the cast fails.

However, PrintWriter and JspWriter are both descended from Writer. Can you change your print() method to accept a Writer instead of an Object, and then use the Writer.write() method within print()? This is common to both classes.

This:

public void print(Writer writer) {
    try {
        writer.write(this.FormattedString());
    } catch (IOException e) {
        // log something...
    }
}

Tony Ennis 2010-10-03 14:26:00

Tried this, but the result seems the same: the method isn't found so it never gets to casting.

Brian Kessler 2010-10-03 14:49:54

Wait, which method isn't found.

Tony Ennis 2010-10-03 14:50:35

As above: org.apache.jasper.JasperException: javax.servlet.ServletException: java.lang.NoSuchMethodError: hu.flux.helper.NameAndAddress.print(Ljavax/servlet/jsp/JspWriter;)V

Brian Kessler 2010-10-03 15:02:37

Insert the code I posted, and comment out the `print(Object out)` method that's currently there. See what happens. Consider putting a main() in your class so you can run tests from the IDE/command line (much faster) or even better, add junit tests.

Tony Ennis 2010-10-03 15:17:43

Still recognizes only PrintWriter, not JspWriter.

Brian Kessler 2010-10-03 15:33:18

Inconceivable! Verify the JspWriter you're using is a subclass of java.io.Writer please.

Tony Ennis 2010-10-03 15:36:22

Answer 6

+1 A:

What you posted should work since both PrintWriter and JspWriter are subclasses of Writer (and of course both are subclasses of Object). Something seems to be wrong either with your test code or with your environment.

Maybe you can try a simplified example and see if that works, then build up from there. I can suggest starting here:

public class Test
{
    public void print(Writer writer) throws IOException
    {
        if (writer == null)
            System.out.println("Null writer");
        else
        {
            writer.write("hello");
            writer.flush();
        }
    }

    public static void main(String args[]) throws IOException
    { 
        Test test = new Test();

        System.out.print("Testing PrintWriter...");
        PrintWriter p = new PrintWriter("d:/temp/pwriter_text.txt");
        test.print(p);
        System.out.print("Tested PrintWriter...");

        System.out.print("Testing JspWriter...");
        JspWriter j = null;
        test.print(j);
        System.out.print("Tested JspWriter...");
    }
}

This should compile and run. The second time test.print() is called, the JspWriter being passed in will be null, but you should not get any NoSuchMethodError. If this works, take the code and test it from your servlet / JSP pages. Hopefully this will help you find the problem.

Grodriguez 2010-10-03 16:30:04

That will compile, but I'll get errors when it hits test.print(j) cause its not instantiated, which it can't be cause its abstract.

Brian Kessler 2010-10-03 16:42:33

@brian Kessler - that's exactly his point. The error you're getting looks to be a _can't find an appropriate method_ error. So Grod's solution, while it won't produce the output you want, _will_ test the idea that the method isn't found. If it still isn't found, then you have a different problem than the one we're trying to solve.

Tony Ennis 2010-10-03 16:47:00

You will not get any errors. As I explained in my answer, when you call `test.print(j)`, `j` will be `null`, but this is checked in the implementation of the `print` method. You should not get any errors or exceptions. The only purpose of this is to demonstrate that the `test` method gets called if a `JspWriter` is passed in.

Grodriguez 2010-10-03 16:48:45

@Tony Ennis: Yes, exactly.

Grodriguez 2010-10-03 16:49:29

+1 good idea...

Tony Ennis 2010-10-03 16:50:30

I _do_ get errors: Exception in thread "main" java.lang.NullPointerException at java.io.Writer.<init>(Unknown Source) at java.io.PrintWriter.<init>(Unknown Source) at java.io.PrintWriter.<init>(Unknown Source) at hu.flux.helper.NameAndAddress.print(NameAndAddress.java:51) at hu.flux.helper.NameAndAddress.main(NameAndAddress.java:97)

Brian Kessler 2010-10-03 16:58:35

@brian Kessler - you have something askew. post your latest and greatest main() if you haven't done so. We can't tell which line is #97, btw. Do you really have doubled doubled-quotes in your code where you specify the PrintWriter output file or is that a formatting issue?

Tony Ennis 2010-10-03 17:01:46

@Brian: Those errors you are getting do not come from the code snippet I suggested. There is no `NameAndAddress` class in my code, and that was intentional. Please try to run the code snippet from my answer. You should not be getting any errors.

Grodriguez 2010-10-03 17:04:06

Actually, the above error is from trying to test in the NameAndAddress class. I didn't expect anything different with the Test class, but I decided to try it anyway. After adding imports, I discovered while it gets errors, the errors are far from what I expected. Eclipse gives me "Launch Error: Editor does not contain a main type" and running from the command line, I get Exception in thread "main" java.lang.NoClassDefFoundError: Could not find the main class: build\classes\hu\flux\helper\Test.class. ... But I see the main class, so I have no idea why it is complaining about it.

Brian Kessler 2010-10-03 17:07:04

@Tony Ennis: I've posted the "latest and greatest" above ... not sure what you mean about the doubled doubled-quoted code...

Brian Kessler 2010-10-03 17:17:10

@Brian: Something seems to be wrong with your environment. Are you able to compile and run a simple "hello world"-like example with a Java class containing a main method?

Grodriguez 2010-10-03 17:21:04

@Grodriguez: I am able to compile and run many Java classes, from a simple "hello world" to the more complex NameAndAddress class we have been discussing. I have never before seen this problem that I'm seeing now with your test method. It has me flabbergasted.

Brian Kessler 2010-10-03 17:31:27

If you can compile a simple hello world then you should certainly be able to compile and run the `Test` class I posted. I can compile and run it without problems. That's why I am suspecting something wrong with your environment, and looks like I'm not the only one. I suggest creating a simple hello world example, perhaps in a class named `Test`, in a new Eclipse project (or you can do without Eclipse, and compile and run from the command line). If that works, replace the `Test` class with the code I posted. If that works, then we'd be getting somewhere.

Grodriguez 2010-10-03 17:34:48

@Grodriguez: Something does seem to be very wrong... While I can still get old classes to work, if I just create a new class that says nothing but public class Test2 { public static void main(String[] args) { System.out.print("Hello! World"); }} I am getting this problem with the main method... going to try turning it off and on again...

Brian Kessler 2010-10-03 17:38:25

OK.. now we're in the right direction.

Grodriguez 2010-10-03 17:42:52

Created a new project and am able to run your test in it... your right, no errors.... this is the output: Testing PrintWriter...Tested PrintWriter...Testing JspWriter...Null writerTested JspWriter...

Brian Kessler 2010-10-03 17:44:37

Very good. So this demonstrates that the `print` method is actually being called for a `JspWriter` argument. In my test code, the `JspWriter` being passed in is `null`, but this will not be the case when this is called from your servlet or JSP. I assume that your question is now solved.

Grodriguez 2010-10-03 17:53:16

@Grodriguez -- Not exactly... After this, I went back to trying to call the method from the JSP pages and there I still get a NoSuchMethod if I rely on print(Writer writer) to catch the reference... so it seems that printer(Writer writer) will catch a JspWriter object from the console, but not a jsp page?

Brian Kessler 2010-10-03 18:10:34

`print(Writer)` will be always called if the parameter passed in is a subclass of `Writer`. If this does not happen in your case, then the problem is clearly somewhere else. I suggest you start from a tiny JSP example (different project, etc etc.) and build up from there.

Grodriguez 2010-10-03 18:17:24

There isn't really much to this code to begin with... I'm just playing with some code from a JSP tutorial (I learn better by engaging than by just staring at or typing code). I've modified the code so that the console will display "--Entering--" timestamp immediately when it is called, even from a JSP page. This code never gets called if print (JspWriter writer) isn't present to catch the call.

Brian Kessler 2010-10-03 18:37:34

Brian, there is something else wrong with your code or setup. As I said (and you were able to verify) the `print(Writer)` method will be called if the argument is a subclass of `Writer`. If this does not work in your tests, then something else is wrong. I'm afraid I cannot help further than this.

Grodriguez 2010-10-03 21:18:24

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Shouldn't a method that receives java.lang.Object as input also receive javax.servlet.jsp.JspWriter as input?

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