verifying that the compiler produces Java 1.5 code

Question

I have a jar file I produced from compiled class files using an ant file with <javac target="1.5" ... /> and I would like to verify that it actually produces 1.5 code. Is there a way to do this?

My version of MATLAB one one of the computers I'm using, uses a JRE 1.5; so it will not run code unless it is JRE 1.5 compatible. It's worked fine with most JAR files I've produced, but I have one that's acting up, with an odd error:

>> s = javaObject('com.example.test.hdf5.Test1');
??? Error using ==> javaObject
No constructor with appropriate signature exists
in Java class com.example.test.hdf5.Test1

even though here's my class, and it has a regular old no-arg constructor:

package com.example.test.hdf5;

import ncsa.hdf.hdf5lib.H5;
import ncsa.hdf.hdf5lib.HDF5Constants;
import ncsa.hdf.hdf5lib.exceptions.HDF5LibraryException;
import ncsa.hdf.object.FileFormat;
import ncsa.hdf.object.h5.H5File;

public class Test1 {
    public Test1 () {}

    public static void main(String args[])
    {
     Test1 test = new Test1();
     if (args.length < 2)
     {

     }
     else if ("h5file".equals(args[0]))
     {
      test.testH5File(args[1]);
     }
     else if ("h5f".equals(args[0]))
     {
      test.testH5F(args[1]);   
     }
    }

    public void testH5File(String filename) {
     H5File file;
     try
     {
      file = (H5File) new H5File().createFile(
        filename, FileFormat.FILE_CREATE_OPEN);
      file.close();
      System.out.println("Success!");
     }
     catch (Exception e)
     {
      throw new RuntimeException(e);
     }
    }

    public void testH5F(String filename) {
     try {
      int id = H5.H5Fopen(filename, 
        HDF5Constants.H5F_ACC_RDONLY, HDF5Constants.H5P_DEFAULT);
      H5.H5Fclose(id);
      System.out.println("Success!");
     }
     catch (HDF5LibraryException e) {
      throw new RuntimeException(e);
     }
     catch (NullPointerException e) {
      throw new RuntimeException(e);
     }
    }
}

Another file produced in the same package + jar file works fine:

package com.example.test.hdf5;

public class Test3 {
    public Test3() {}

    private int x=0;

    public int foo() { return ++this.x; }
}

and I'm wondering if there's something that's screwing up the compiler's 1.5-ness by importing a library that may not be 1.5-compatible.

---

update: both my Test1 and Test3 classes are 1.5 (major=0, minor=49 as per javap -v). I added a Test2.java which is the exact same as Test1 but with the body of the methods commented out, so it has the same signatures. I get the following with javap -s:

C:\proj\java\test-hdf5\dist>javap -s -classpath test-hdf5.jar com.example.test.hdf5.Test1
Compiled from "Test1.java"
public class com.example.test.hdf5.Test1 extends java.lang.Object{
public com.example.test.hdf5.Test1();
  Signature: ()V
public static void main(java.lang.String[]);
  Signature: ([Ljava/lang/String;)V
public void testH5File(java.lang.String);
  Signature: (Ljava/lang/String;)V
public void testH5F(java.lang.String);
  Signature: (Ljava/lang/String;)V
}


C:\proj\java\test-hdf5\dist>javap -s -classpath test-hdf5.jar com.example.test.hdf5.Test2
Compiled from "Test2.java"
public class com.example.test.hdf5.Test2 extends java.lang.Object{
public com.example.test.hdf5.Test2();
  Signature: ()V
public static void main(java.lang.String[]);
  Signature: ([Ljava/lang/String;)V
public void testH5File(java.lang.String);
  Signature: (Ljava/lang/String;)V
public void testH5F(java.lang.String);
  Signature: (Ljava/lang/String;)V
}


C:\proj\java\test-hdf5\dist>javap -s -classpath test-hdf5.jar com.example.test.hdf5.Test3
Compiled from "Test3.java"
public class com.example.test.hdf5.Test3 extends java.lang.Object{
public com.example.test.hdf5.Test3();
  Signature: ()V
public int foo();
  Signature: ()I
}

I guess there's just something really weird going on in the HDF5 library JHDF5.jar that causes MATLAB to reject my Test1 class.

update 2 >:( >:( >:( The JHDF5.jar file has version 50 (JRE1.6), so that's probably what's causing me to lose. Phooey to Matlab for not producing a meaningful error message, and Phooey to HDF5 for compiling with JRE1.6 instead of 1.5 or earlier; I'm very doubtful they're using any of the 1.6 features. I'll file a bug report.

Answer 1

Try methodsview('com.example.test.hdf5.Test1') to see exactly what Matlab thinks your constructor expects.

Answer 2

To determine the Java bytecode version of a class file, use javap -v <classname>. This will output something like

minor version: 0
major version: 49

Major version 49 is Java platform version 1.5 (48 is 1.4, 50 is 1.6)

Answer 3

This may be a shot in the dark, but did you make any modifications to Test1 while MATLAB was still running? If so, you may have to use the CLEAR command:

clear java

From the CLEAR documentation:

Issue a clear java command after modifying any files on the Java dynamic class path.

Answer 4

This site indicates that

Typically, you do not need to use javaObject. Use the default MATLAB syntax for instantiating a Java class instead. Use javaObject for the cases described above.

So, instead of using javaObject, which creates instances reflectively, you can, if you know the class you are going to create, just use the constructor syntax:

s = com.example.test.hdf5.Test1()

Does this work?