[DrJava] [ drjava-Bugs-1926776 ] DynamicJava throws NullPointerException

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Bugs item #1926776, was opened at 2008-03-26 22:48
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Category: DynamicJava
Group: None
Status: Open
Resolution: None
Priority: 9
Private: No
Submitted By: Mathias Ricken (mgricken)
Assigned to: Dan Smith (dlsmith)
Summary: DynamicJava throws NullPointerException

Initial Comment:
At the end of the following interactions, DynamicJava throws a NullPointerException (see Stack Trace 1 below), even though it should print "null".

Example 1:

Welcome to DrJava.  Working directory is /Users/Shared/drjava
> import java.lang.ref.WeakReference;
> WeakReference<String> wref
> String s = "foo"
> wref = new WeakReference<String>(s);
> wref.get()
"foo"
> wref.clear()
> wref.get()

After clearing the weak reference, WeakReference.get() returns null. The return value of wref.get() is null; therefore, the second call to wref.get(), the one that throws the exception, should produce the same result as 

Welcome to DrJava.  Working directory is /Users/Shared/drjava
> null
null

Furthermore, I have noticed additional strange behavior after wref.clear(). For example:

Example 2:

Welcome to DrJava.  Working directory is /Users/Shared/drjava
> import java.lang.ref.WeakReference;
> WeakReference<String> wref
> String s = "foo"
> wref = new WeakReference<String>(s)
java.lang.ref.WeakReference@8523a0
> wref.get()
"foo"
> wref.clear()
> Boolean boo = new Boolean(wref.get()==null)
> boo
Static Error: Undefined name 'boo'

Why is 'boo' not recognized?

When I ran the example 2 on a "fresh" DrJava, one that had not experienced the NullPointerException from example 1, I got another NullPointerException, the one shown in Stack Trace 2 below. Apparently "Reset Interactions Pane" is not enough to completely reset DynamicJava; the InteractionsPane never recuperated from the NullPointerException in example 1.

Another example:

Example 3:

Welcome to DrJava.  Working directory is /Users/Shared/drjava
> import java.lang.ref.WeakReference;
> WeakReference<String> wref
> String s = "foo"
> wref = new WeakReference<String>(s)
java.lang.ref.WeakReference@8523a0
> wref.get()
"foo"
> wref.clear()
> (wref.get()==null)
> 

Why does the comparison not produce any output? It should produce

> (wref.get()==null)
true
>

Again, if I ran this on a "fresh" DrJava, I got the NullPointerException shown in Stack Trace 2.

A final example:

Example 4:

Welcome to DrJava.  Working directory is /Users/mgricken/Documents/Java
> import java.lang.ref.WeakReference;
> String s = "foo"
> WeakReference<String> wref = new WeakReference<String>(s)
> wref.get()
"foo"
> wref.clear()
> boolean b = true
> b
true
> b = (wref.get()!=null)
> b
true
> 

The value of b did not change. The result at the end, of course, should have been

> b = (wref.get()!=null)
> b
false
> 

Again, if run on a "fresh" DrJava, I got the NullPointerException in Stack Trace 2.

------------------------------

Stack Trace 1:

edu.rice.cs.util.UnexpectedException: java.lang.NullPointerException
	at edu.rice.cs.drjava.model.repl.newjvm.MainJVM$ResultHandler.forUnexpectedException(MainJVM.java:790)
	at edu.rice.cs.drjava.model.repl.newjvm.MainJVM$ResultHandler.forUnexpectedException(MainJVM.java:744)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpretResult$UnexpectedExceptionResult.apply(InterpretResult.java:95)
	at edu.rice.cs.drjava.model.repl.newjvm.MainJVM.interpret(MainJVM.java:190)
	at edu.rice.cs.drjava.model.repl.RMIInteractionsModel._interpret(RMIInteractionsModel.java:72)
	at edu.rice.cs.drjava.model.repl.InteractionsModel.interpret(InteractionsModel.java:181)
	at edu.rice.cs.drjava.model.repl.InteractionsModel.interpretCurrentInteraction(InteractionsModel.java:164)
	at edu.rice.cs.drjava.ui.InteractionsController$10$1.run(InteractionsController.java:455)
Caused by: java.lang.NullPointerException
	at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:44)
	at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:103)
	at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:29)
	at koala.dynamicjava.tree.ExpressionStatement.acceptVisitor(ExpressionStatement.java:106)
	at edu.rice.cs.dynamicjava.interpreter.Interpreter.evaluate(Interpreter.java:87)
	at edu.rice.cs.dynamicjava.interpreter.Interpreter.interpret(Interpreter.java:48)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:253)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:229)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:294)
	at sun.rmi.transport.Transport$1.run(Transport.java:153)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.rmi.transport.Transport.serviceCall(Transport.java:149)
	at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:466)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:707)
	at java.lang.Thread.run(Thread.java:613)

Caused by:
java.lang.NullPointerException
	at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:44)
	at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:103)
	at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:29)
	at koala.dynamicjava.tree.ExpressionStatement.acceptVisitor(ExpressionStatement.java:106)
	at edu.rice.cs.dynamicjava.interpreter.Interpreter.evaluate(Interpreter.java:87)
	at edu.rice.cs.dynamicjava.interpreter.Interpreter.interpret(Interpreter.java:48)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:253)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:229)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:294)
	at sun.rmi.transport.Transport$1.run(Transport.java:153)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.rmi.transport.Transport.serviceCall(Transport.java:149)
	at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:466)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:707)
	at java.lang.Thread.run(Thread.java:613)

------------------------------

Stack Trace 2:

edu.rice.cs.util.UnexpectedException: java.lang.NullPointerException
	at edu.rice.cs.drjava.model.repl.newjvm.MainJVM$ResultHandler.forUnexpectedException(MainJVM.java:787)
	at edu.rice.cs.drjava.model.repl.newjvm.MainJVM$ResultHandler.forUnexpectedException(MainJVM.java:741)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpretResult$UnexpectedExceptionResult.apply(InterpretResult.java:95)
	at edu.rice.cs.drjava.model.repl.newjvm.MainJVM.interpret(MainJVM.java:190)
	at edu.rice.cs.drjava.model.repl.RMIInteractionsModel._interpret(RMIInteractionsModel.java:73)
	at edu.rice.cs.drjava.model.repl.InteractionsModel.interpret(InteractionsModel.java:212)
	at edu.rice.cs.drjava.model.repl.InteractionsModel.interpretCurrentInteraction(InteractionsModel.java:193)
	at edu.rice.cs.drjava.ui.InteractionsController$10$1.run(InteractionsController.java:459)
Caused by: java.lang.NullPointerException
	at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:44)
	at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.visit(ExpressionEvaluator.java:227)
	at koala.dynamicjava.tree.EqualExpression.acceptVisitor(EqualExpression.java:72)
	at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:39)
	at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:103)
	at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:29)
	at koala.dynamicjava.tree.ExpressionStatement.acceptVisitor(ExpressionStatement.java:106)
	at edu.rice.cs.dynamicjava.interpreter.Interpreter.evaluate(Interpreter.java:87)
	at edu.rice.cs.dynamicjava.interpreter.Interpreter.interpret(Interpreter.java:48)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:250)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:227)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:294)
	at sun.rmi.transport.Transport$1.run(Transport.java:153)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.rmi.transport.Transport.serviceCall(Transport.java:149)
	at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:466)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:707)
	at java.lang.Thread.run(Thread.java:613)

Caused by:
java.lang.NullPointerException
	at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:44)
	at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.visit(ExpressionEvaluator.java:227)
	at koala.dynamicjava.tree.EqualExpression.acceptVisitor(EqualExpression.java:72)
	at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:39)
	at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:103)
	at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:29)
	at koala.dynamicjava.tree.ExpressionStatement.acceptVisitor(ExpressionStatement.java:106)
	at edu.rice.cs.dynamicjava.interpreter.Interpreter.evaluate(Interpreter.java:87)
	at edu.rice.cs.dynamicjava.interpreter.Interpreter.interpret(Interpreter.java:48)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:250)
	at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:227)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:294)
	at sun.rmi.transport.Transport$1.run(Transport.java:153)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.rmi.transport.Transport.serviceCall(Transport.java:149)
	at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:466)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:707)
	at java.lang.Thread.run(Thread.java:613)

System Properties:
DrJava Version 20080327-0151
DrJava Revision 4418

drjava.debug.port = 55787
java.runtime.name = Java(TM) 2 Runtime Environment, Standard Edition
sun.boot.library.path = /Applications/Dev/YourKit Java Profiler 6.0.12.app/bin/mac:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Libraries
java.vm.version = 1.5.0_13-121
awt.nativeDoubleBuffering = true
gopherProxySet = false
java.vm.vendor = "Apple Computer, Inc."
java.vendor.url = http://apple.com/
path.separator = :
java.vm.name = Java HotSpot(TM) Client VM
file.encoding.pkg = sun.io
sun.java.launcher = SUN_STANDARD
user.country = US
sun.os.patch.level = unknown
java.vm.specification.name = Java Virtual Machine Specification
user.dir = <anonymized user.dir>
java.runtime.version = 1.5.0_13-b05-241
java.awt.graphicsenv = apple.awt.CGraphicsEnvironment
java.endorsed.dirs = /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/lib/endorsed
os.arch = i386
java.io.tmpdir = /tmp
line.separator = "\u000a"
java.vm.specification.vendor = Sun Microsystems Inc.
os.name = Mac OS X
apple.laf.useScreenMenuBar = true
sun.jnu.encoding = MacRoman
java.library.path = /Applications/Dev/YourKit Java Profiler 6.0.12.app/bin/mac:.:/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java
java.specification.name = Java Platform API Specification
java.class.version = 49.0
sun.management.compiler = HotSpot Client Compiler
os.version = 10.4.11
user.home = <anonymized user.home>
user.timezone = America/Chicago
java.awt.printerjob = apple.awt.CPrinterJob
file.encoding = MacRoman
java.specification.version = 1.5
java.class.path = drjava-15.jar:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/.compatibility/14compatibility.jar:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/.compatibility/14compatibility.jar
user.name = <anonymized user.name>
apple.awt.graphics.UseQuartz = true
java.vm.specification.version = 1.0
java.home = /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home
sun.arch.data.model = 32
user.language = en
java.specification.vendor = Sun Microsystems Inc.
awt.toolkit = apple.awt.CToolkit
java.vm.info = mixed mode, sharing
java.version = 1.5.0_13
java.ext.dirs = /Library/Java/Extensions:/System/Library/Java/Extensions:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/lib/ext
sun.boot.class.path = /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/classes.jar:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/ui.jar:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/laf.jar:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/sunrsasign.jar:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/jsse.jar:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/jce.jar:/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/charsets.jar
java.vendor = Apple Computer, Inc.
file.separator = /
java.vendor.url.bug = http://developer.apple.com/java/
sun.io.unicode.encoding = UnicodeLittle
sun.cpu.endian = little
java.rmi.server.hostname = 127.0.0.1
mrj.version = 1040.1.5.0_13-241
sun.awt.exception.handler = edu.rice.cs.drjava.ui.DrJavaErrorHandler
sun.cpu.isalist = 

#DrJava configuration file
#Wed Mar 26 22:33:23 CDT 2008
font.line.numbers = Monaco-10
lineenum.enabled = true
key.delete.next = shift DELETE
key.delete.previous = shift BACK_SPACE
find.replace.match.case = false
interactions.exit.prompt = false
open.folder.recursive = true
recent.files = [<anonymized user.home>/Documents/Java/WeakRef.java,<anonymized user.home>/Documents/Java/Tiny.java,<anonymized user.dir>/src/edu/rice/cs/drjava/config/EagerFileProperty.java,<anonymized user.dir>/src/edu/rice/cs/drjava/CodeStatus.java,<anonymized user.dir>/src/edu/rice/cs/drjava/Version.java]
recent.projects = [<anonymized user.dir>.pjt,<anonymized user.home>/Documents/Semester 8 Spring 2008/COMP 312/hw1/solution/hw1sol.pjt,<anonymized user.home>/Documents/Research/Concutest/MSThesis.defense/RCSC-20071004/listings/rscs-20071004.pjt,<anonymized user.home>/Documents/Semester 8 Spring 2008/COMP 312/hw1/assignment/hw1.pjt]
window.height = 776
window.width = 1276
window.x = 0
window.y = 22
doc.list.width = 277
last.dir = <anonymized user.home>/Documents/Java/WeakRef.java
last.interactions.dir = <anonymized user.home>/Documents/Java
master.jvm.xmx = 512
dialog.clipboard.history.state = 425 400 425 210
dialog.completeword.scan.class.files = true
dialog.completeword.javaapi = true
find.replace.focus.in.defpane = true

Used memory: about 12.61 megabytes
Free memory: about 767.16 kilobytes
Total memory: about 13.36 megabytes
Total memory can expand to: about 508.06 megabytes

----------------------------------------------------------------------

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[DrJava] [ drjava-Bugs-1926776 ] DynamicJava throws NullPointerException

A lightweight programming environment for Java

[DrJava] [ drjava-Bugs-1926776 ] DynamicJava throws NullPointerException