See the Toad Help File excerpt below:
Troubleshooting Java Debugging
The Java debugger uses the same basic user interface as the PL/SQL debugger. Because it uses the Oracle package DBMS_DEBUG_JDWP to access the debugging features, it is entirely Oracle-oriented. This means that there are Oracle-imposed limitations on the debugging procedures you can use through Toad.
The Java debugger does not support triggers.
Oracle 9iR2 Issues
The debugger package (DBMS_DEBUG_JDWP) was first released with Oracle 9iR2. In this release there are some limitations.
Stepping into Code
When the debugger steps into Java source, sometimes ORA-03113 errors will be generated by the database.
To continue debugging
From Toad Options|Editor|Debugging, clear the Allow stepping into Java Source box.
Note: You will not be able to step into Java Source code with this box cleared.
Oracle 10g Issues
Debugger jumps over bulleted lines of code
On a Windows server, the debugger may jump over bulleted lines of code in the Java Source. There is no known work around at this time.
Debugger gutter line execution bullets not visible
In JDWP debugging, the line map cannot be retrieved until the debugger is called and initialized. This will keep the gutter bullets from displaying until debugging or execution starts.
Directing Output to the DBMS Output window
On a line that uses the dbms_java.set_output command, if you are using 9iR2 on a Windows OS, Oracle may return ORA-3113 errors.
When you trace into code that calls the DBMS Output window, errors are generated.
Tracing into System.out.println
Tracing into System.out.println can generate ORA-3113 errors.