Modifying the dynamic Java classpath when Java objects are loaded

I have a problem updating MATLAB's dynamic Java classpath when there are variables in the workspace that point to Java objects whose classes are on the dynamic classpath.

I have two Java test classes matlabtest.Test1 and matlabtest.Test2:

 >> ls java1\matlabtest\

 .            ..           Test1.class  

 >> ls java2\matlabtest\

 .            ..           Test2.class

Adding the first directory to the classpath and instantiating matlabtest.Test1 is not a problem:

 >> javaaddpath('java1')
 >> test1 = matlabtest.Test1()

 test1 =

 matlabtest.Test1@606032

However, now adding the second directory and instantiating matlabtest.Test2 does not work:

 >> javaaddpath('java2')
 Warning: Objects of matlabtest/Test1 class exist - not clearing java 
 > In javaclasspath>doclear at 379
   In javaclasspath>local_javapath at 197
   In javaclasspath at 119
   In javaaddpath at 69
>> test2 = matlabtest.Test2()
??? Undefined variable "matlabtest" or class "matlabtest.Test2".

A workaround:

 >> clear test1;
 >> clear java;
 >> test2 = matlabtest.Test2()

 test2 =

 matlabtest.Test2@fcc070

However, this workaround is not possible in my case (the relevant objects belong to another product and I cannot simply clear them without upsetting my users). I also cannot MATLAB's static Java classpath (the exact directories are only known at runtime).

So: How can I create Java objects from truly dynamic locations?

Answer 1

[Jacob Halbrooks]

Steve Lord answered a similar question on the MATLAB Newsgroup. I think his answer also applies to your question. Here is what he wrote:

When you call JAVAADDPATH, it calls CLEAR with the flag 'java' as part of its execution.

http://www.mathworks.com/help/techdoc/ref/javaaddpath.html

As documented on the reference page for CLEAR, "clear java" behaves like "clear all" in that it clears all variables from the base workspace.

http://www.mathworks.com/help/techdoc/ref/clear.html

If you want to recover those variables you need to either SAVE them before calling JAVAADDPATH and LOAD them afterwards or reexecute the code that generated those variables.

[Florian Brucker]

My problem is not that |clear java| performs a |clear all|. My problem is that |clear java| is not performed at all since there are variables in the workspace which point to instances of Java objects and that it is impossible to modify the dynamic classpath without calling |clear java| (explicitely or implicitely via |javaaddpath|).

[Jacob Halbrooks]

I think that is all correct; you ultimately need to clear your existing Java variables to add a new Java path. As a result, you need to follow one of the work-arounds suggested by Steve to recover those variables.