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The Pragmatics Of Java Debugging, by Joe Winchester & Arthur Ryman

Essential to the development of complex systems are tools that help the developer locate, analyze, and fix problems. Debuggers provide support for this by letting a developer inspect the internal state of a program at runtime, as well as suspend and resume execution statement by statement.

The originators of the Java programming language defined a debugging architecture, but since its conception Java has advanced into new areas of deployment topologies and optimization technologies that present a further set of problems. This article covers some of the background behind these issues as well as the activity in the Java community to provide solutions. Examples of debugging solutions are drawn from the IBM VisualAge for Java integrated development environment (IDE), al-though the issues are applicable to other environments as well.

Debugger Basics
Java source code is written in .java files. The source is compiled into a .class file - bytecodes interpreted by a Java Virtual Machine (JVM). The JVM dynamically loads class files by searching the program's classpath. The purpose of the JVM is to execute the bytecodes and let the user navigate and execute their application. It does this by translating the bytecodes into a set of lower-level machine instructions that are specific for the platform on which the JVM is running. This level of indirection is what allows different JVMs to be written for different hardware or operating system APIs, while letting the bytecode class files meet the promise of "write once - run anywhere."

Dynamic interpretation of the bytecodes into machine instructions gives Java the advantages of portability and operating system neutrality, but it comes at the price of reduced performance.

When a program gets compiled into bytecodes, the compiler focuses on making the bytecodes executable by the target JVM. However, if the program is going to be debugged, the developer needs to be able to trace the bytecodes back to their source. Java is a very dynamic language, so the physical layout of a class is not determined at compile-time but is actually generated by the JVM when it loads the class. Therefore all class, method, and field names are preserved in the class file, which also includes the name of the source file and a line number table that maps ranges of bytecodes to their corresponding line in the source file.

The JVM provides an API for setting breakpoints and manipulating the stack frame. This is the API that the Java Development Kit (JDK) debugger uses and it also surfaced in the package sun.tools.debug. The JDK debugger is a command-line debugger that, when executed (with the jdb command), lets the developer perform basic tasks such as inserting breakpoints on statement lines, catching thrown exceptions when they're raised, and printing and advancing the stack trace. The jdb program is limited in its functionality due to its command line interface, but there are a number of good debuggers that have graphical interfaces to let the developer view and monitor the program while debugging it. The IBM Distributed Debugger and Symantec Visual Cafe are two such debuggers, as is the integrated debugger that comes with the VisualAge for Java IDE.

To reduce the size of the class file, the javac compiler normally removes symbolic information not required at runtime, such as method argument and local variable names. The debug API returns generated names such as arg_1, instead of the original argument name given by the developer. If you need to debug a program with the original names the javac option -g can be used.

Writing a Program to Aid Debugging
When developers write programs in a high-level language, they concentrate on making maintainable, error-free, and extensible source code. Another often overlooked goal is to write code that's debuggable. Certain programming practices can help to debug a program such as using temporary variables to store the result of intermediate method calls. Often a developer uses a temporary variable to store the result of a call that's used more than once in the method. If the method result is used directly on the next line, the statements are typically chained together. If a temporary variable is used instead, at debug time you can see the intermediate result. The following code shows an example of chaining method calls together:

anObject.setSomething(anotherObject.getSomething() + yetAnotherObject.getSomethingElse() );
In the debugger, you can't see the result of getSomething()or getSomethingElse(). The code could be rewritten as:
Object something = anotherObject.getSomething();
Object somethingElse = yetAnotherObject.getSomethingElse();
anObject.setSomething(something + somethingElse);
When you single-step through this code, you can see the result of each method result in turn. The debugger API lets you insert breakpoints at the start of each statement, so writing code that doesn't combine many expressions into a single statement also provides more target points to insert a potential breakpoint.

Debugging a Program Running Inside a JVM
The typical life cycle for a Java program is the developer writes some code, compiles it, and then tests it by executing the compiled class file in a JVM. To do this the developer must first compile the program to allow debugging, and then use a debugger that calls the Java debug API to control the JVM program execution.

When the JVM raises an exception or hits a breakpoint, the debugger visually shows a stack trace of the program so developers can inspect the contents of the program variables. The Distributed Debugger also shows the source code if it's available for the debugger to use (see Figure 1).

Figure 1

When the debugger has halted program execution and you've inspected the program state, you can use the step functions to continue execution statement by statement, or just resume the program until it reaches the next uncaught exception or breakpoint. This lets you understand the dynamics of the program execution and locate errors. Once you've located a problem you might see the errant code and want to change some of the source and test the change. To do so typically requires that you exit the program, recompile the .java source file, and rerun the program with the new source. In an environment with fast turnaround time where you're trying to quickly test and fix code, this stop-change-compile-start cycle can be time-consuming.

In addition, if the program is being debugged in a server environment, replacing the server class files might be difficult. If the server isn't designed for development it might place locks on jar files, and there may be no way to reload a modified class, requiring you to restart the server, then re-create the error condition. Servers may take a long time to restart and, if the server is a production server, downtime may be difficult to obtain. Some servlet engines, such as WebSphere, can reload a servlet, but if the servlet uses other classes such as JavaBeans or EJBs, and you modify them, you may still need to restart the server.

Debugging a Program Running Inside an IDE
Developers writing code they wish to test can compile their source into class files and debug it in a JVM as described above. However, the turnaround time of recompilation, debug, analyze, fix source, and compilation can hinder overall development productivity. VisualAge for Java is a development environment that lets developers write and debug code without any explicit recompilation or program halting. It has a virtual machine that's tightly integrated with the development environment and that allows recompilation into an already running program, as well as incremental compiling so that source and bytecodes are always kept in step. This means that when a breakpoint is reached, the developer can just modify the source and click save. The incremental compiler reports any problems, and if there are none the program resumes execution at the top of the new method.

Developers can also execute an ad hoc piece of java code or modify program variables from within a running program. When the program is suspended in the debugger, the developer can select any method in the call stack and tell the debugger to drop to this frame. This is rather like a rewind button and is useful if the debugger gets invoked by an exception and the developer needs to go back a few method calls and retrace some steps to see the root cause of a problem. NullPointerException is a good example of where the exception is usually thrown too late; the problem isn't the method call on a null variable; it's the earlier method call that was supposed to set the variable to a non-null value that needs debugging. Figure 2 shows the integrated debugger that comes with VisualAge for Java.

Figure 2

The VisualAge for Java debugger works on a Java source running within its own virtual machine. The virtual machine is part of the development environment, which means that it can't be easily replaced. To debug code executing in a different JVM, you can use the Distributed Debugger, but this sacrifices the ability to execute ad hoc java code, rewind program execution, change variable contents, and modify source inside a running program.

VisualAge for Java also supports this fast code-debug-fix cycle for the WebSphere environment. A stripped-down version of the WebSphere environment, also known as the WebSphere Test Environment, is shipped as part of VisualAge for Java. The WebSphere Test Environment executes as part of VisualAge for Java's internal virtual machine, which means you can test and debug server components such as servlets or EJBs using the techniques described previously. When you've completed the first pass of testing your code, you can export and deploy it in a true WebSphere environment where you can further debug it with the Distributed Debugger. In addition to hosting the WebSphere Test Environment (which emulates the true WebSphere environment inside the IDE), you can also load other environments such as Jakarta Tomcat and New Atlanta ServletExec into VisualAge for Java, which lets you do integrated development and debugging inside the IDE before deploying the server code into a production environment.

Java Debug Topology
The Distributed Debugger lets you debug a Java program that's been compiled for debugging. The remote debugger runs on a client and can debug Java programs that are either executing on the same client (local) or on another machine (remote), such as a server. These are the two most common scenarios for debugging: the Java program is a traditional client program such as a two-tier fat client application, or the Java program is a component, such as a servlet or an EJB, executing as part of a remote application server.

To allow the debugger to work with both local and remote programs, it's split into two portions: the debugger user interface that the developer uses to view and control the program being debugged, and the debug engine that the interface talks to and in turn debugs the JVM. This separation of interface from engine, which is part of the VisualAge for Java Distributed Debugger, allows multiple engines to be controlled from the same interface. This means that developers can seamlessly debug Java on their client through remote calls on a server such as an IBM OS/390 or an AS/400, or a Microsoft Windows NT server.

Cross-Language Debugging
Although Java promises to be the panacea for all programming problems, a complex application will have code that's written in other languages. This code could be a dynamic link library (DLL) written in C, or some JavaScript in an HTML page. The Distributed Debugger uses a standard interface between the user interface and the debug engines that allows other languages to be debugged from the same user interface. The complete list of supported languages varies by platform, but the list includes Java, C, C++, Fortran, Cobol, PL/1, and RPG. Having a single user interface that can debug multiple languages on multiple servers lets the developer trace through an execution scenario and narrow in on the errant code or condition, irrespective of which part of the system or server it's on.

Translated Java Programs
When the Java language was first created, Java programs were running in only a few environments. Since then the use of Java has spread and Java programs can be generated from other languages such as JavaServer Pages and SQLJ. JSPs are used on the server and allow Web servers to generate dynamic HTML page content and send it to the client, while SQLJ allows SQL statements to be embedded in Java programs, and used wherever the database access is performed. Both JSP and SQLJ work by translating the developer's code into a Java program that's then compiled into executable bytecodes. For a JSP this is done by creating a servlet that contains that JSP's Java code. This extra step means that the program actually being debugged is not directly associated with the source the developer wrote.

A number of vendors, such as IBM and Oracle, recognized the need to let the developers debug the source they wrote rather than the translated source. In the C language, a similar problem is caused by the preprocessor, but the source line mapping is preserved through the use of the #line pragma. However, the Java Language Specification did not include a preprocessor or the equivalent of the #line pragma, so each vendor had to implement a proprietary way to preserve line numbers, typically by inserting comments into the translated Java source. Java Specification Request (JSR) 45 defines a standard line-mapping table to preserve the correspondence from the original source (e.g., JSP or SQLJ) to the translated Java source, which could then be read by the javac compiler or another program to postprocess the line number information stored in the class file.

After a class file has been modified to reflect the original source file name and line number information, standard debuggers can be used to debug the untranslated source. The difference can be seen in Figures 3 and 4. Figure 3 shows the VisualAge for Java debugger with the breakpoint in the JSP source statement, whereas Figure 4 shows the breakpoint in the Java servlet source that the JSP was translated into.

Figure 3

Figure 4

Server Dynamics
A complex application can often span several remote environments in which a servlet in one server can call an EJB in a container on another server, that in turn can execute an RMI call to yet another server. It's this kind of topology that the Distributed Debugger solves with its Object Level Trace (OLT), as it can start a debug engine on each server and let the interface seamlessly debug across the different boundaries. Server environments that the OLT supports include the IBM Component Broker and the WebSphere Application Server.

The Distributed Debugger can be opened from within the OLT, which lets you examine and profile program flow, as well as perform the traditional debugging tasks of setting breakpoints, inspecting program variables, and controlling program flow. The OLT is useful for viewing the dynamics of a distributed application, because it includes a trace server that receives trace messages from each process involved in the distributed application. The individual trace messages are assembled so you can follow the sequence of events from process to process.

Another problem that exists with debugging server programs is that when a problem occurs in the server, it's often difficult to re-create the same problem in another environment. To do so requires staging environments that must mimic the server environment in terms of hardware resources as well as runtime conditions such as server load. The reality is that some problems that occur in a production environment can never be re-created in a development environment. The Distributed Debugger can be configured so that a problem on a server can actually call back and invoke a client to let the developer debug the server environment from their console. The ability to have a fully functional debugger opened on a server program that's thrown an exception gives you a much greater view of the problem than just a textual stack trace.

Just-In-Time Optimization
Because bytecode interpretation isn't as fast as native machine code execution, a number of optimization technologies have sprung up. One of these is Just-In-Time or JIT compilation. JIT compiles each method to machine code the first time it's called, and caches the result so it doesn't have to recompile the next time. The default for the JDK JVM is to enable JIT, although you can use the -nojit option to disable it. JIT compilation further obscures the relationship between the Java source code and the machine instructions being executed. When debugging code that's been JIT optimized, the JVM must map the machine code back to the bytecode so the debugger can display the corresponding source.

Many commercial JVMs aren't able to cope with this and disable JIT compilation when they execute a program in debug mode. The IBM and Sun JDKs both disable JIT in debug mode, but the VisualAge for Java IDE does not. Since interpreted bytecodes run 10-20 times slower than machine code, debugging can become a tedious process, especially if you're debugging a complex program such as a Web application server. This is another factor that makes debugging in the WebSphere Test Environment more productive because the VisualAge for Java IDE's JVM always performs JIT optimization.

Disabling JIT can also lead to problems, because it means you're not actually debugging the same program that you'd otherwise be executing. Subtle bugs such as those caused by race conditions or nonsynchronized object lock conflicts might not appear in the slower running debugged program.

This is rather like the quantum physics problem of Schroendinger's cat. In this thought experiment the physicist is presented with a closed box that contains a cat and a cannister of poison gas that's released when a radioactive atom decays. Since the atom exists in a superposition of decayed and nondecayed quantum states, the cat exists in a superposition of dead and alive states. Only by observing the system is it forced into a definite state. Is the cat dead or alive? For the physicist there's no real way of knowing because once the box is opened, the act of observing the cat may be the act that kills it. Just as with quantum mechanics where the observer of the cat affects the system being observed, so too can the debugger affect the system being debugged.

Static Optimization
Another way in which Java programs can be optimized is by static optimization. A static optimizer analyzes a whole Java program and attempts to create a more efficient version. There are two types of static optimization: bytecode and machine code.

Bytecode optimization is where the optimizer applies heuristics to rewrite the bytecode output of the Java compiler to make it more efficient for the JVM to process. Techniques used are:

  • Inline method calls to avoid the overhead of locating, executing, and leaving a method
  • Changing an object's type, retyping from an interface to an implementation if the interface has only one implementer (thereby allowing direct binding of the method call to the method location)
  • Making fields final that the optimizer determines are not modified after they've been initialized

The output of bytecode optimizers is Java bytecodes, so they can still run within any JVM. Examples of bytecode optimizers are Dash O and Jove. Since static optimizers alter the bytecodes, the mapping from bytecodes to source code may also be affected, which could prevent debugging. In general, debugging should be performed on unoptimized code if possible.

Machine code optimization is when the Java bytecodes are translated into machine code at compile time rather than at JVM execution time. This is analogous to how languages such as C and Fortran are compiled, but with Java, the optimizer translates the bytecodes into machine code. Examples of such static optimizers are TowerJ from Tower and the IBM High Performance Java Compiler (HPJ) that comes with VisualAge for Java.

Once HPJ has optimized a program, the JVM is no longer used to execute the bytecodes since HPJ generates a self-contained executable program. This has the performance benefits of a natively compiled language, but it also means the program will run only on a platform for which the HPJ can generate an optimized program. These platforms include Windows, OS/2, AIX, AS/400, and OS/390. Because the HPJ bypasses the JVM, the standard Java debug APIs will no longer work. Instead, the Distributed Debugger, which handles both bytecodes and machine code, must be used to debug HPJ-optimized programs.

Java Platform Debug Architecture
JDK 1.3 includes significant enhancements to the debug API via its Java Platform Debug Architecture (JPDA). The JPDA separates the debug API into three distinct layers:

  • Java VM Debug Interface (JVMDI): Provides an interface to allow a VM to be debugged
  • Java Debug Wire Protocol (JDWP): Provides an API into a JVMDI
  • Java Debug Interface (JDI): Lets a front end sit on top of the JDWP
This three-tier architecture tackles the problem of letting tool developers write debuggers that run portably across platforms, JVM implementations, and JDK versions.

At the highest level, someone writing a graphical debugger can hook into the JDI instead of the API previously described in the sun.tools.debug package. This means that the debugger will automatically work with all JVMs and platforms that Sun supports. If another company writes a different JVM, they'll also write their own JDWP implementation, which means the graphical debugger will be able to debug the other company's JVM. If the tool developer writes a debugger that's not written in Java, rather than program to the JDI layer they can program to the lower layers such as JDWP. The JDPA will work well if vendors who write their own JVMs and others who write their own debuggers all program to this interface. This consistency will mean that the debug experience for a developer working in a complex server environment with heterogeneous JVMs and JDK levels will become much more pleasant than it currently is.

This article has covered some of the issues that surround debugging Java programs, a topic of vital interest to professional developers. These issues range from problems created by JIT and static optimizers and new Java source variants such as JSP and SQLJ, to the issues surrounding distributed programs composed of servlets and EJBs that span multiple processes and servers. Java, however, has demonstrated a remarkable ability to adapt and find solutions to such problems. The Java Community Process through JSR 45 is addressing the need for a standard mechanism to debug translated source. The new JPDA in JDK 1.3 provides a common API that both debugging tool providers and JVM builders can program to.

We've provided some background into the problems of debugging Java programs, as well as some of the currently available solutions and some you should see in the near future. We welcome all feedback.


  1. JSR 45: http://java.sun.com/aboutJava/communityprocess/jsr/jsr_ 045_debug.html
  2. Java Platform Debug Architecture: http://java.sun.com/products/ jdk/1.3/docs/guide/jpda/architecture.html
  3. IBM VisualAge for Java and IBM Distributed Debugger: www7.software.ibm.com/vad.nsf
  4. IBM WebSphere: www-4.ibm.com/software/webservers

Author Bios
Joe Winchester is a software developer at the IBM Research Triangle Park lab in North Carolina working on development tools for WebSphere including VisualAge for Java. He is currently working on composition editors that let programmers construct GUIs for different runtime environments as well as write program logic by connecting JavaBeans together.
[email protected]

Arthur Ryman is a senior technical staff member at the IBM Toronto Lab where he is currently working on the XML and Web Services Development Environment, a new tool suite for developing Java, SQL, and XML-based Web Services that support SOAP, WSDL, and UDDI. Previously he worked on VisualAge for Java, specializing in tools for developing servlets and JSPs.
[email protected]


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