What is a debugger?

In this section, you will get a brief overview of what a debugger is and the features that it typically offers. You will also learn how the Scala debugger API is related to the Eclipse debugger (described below).


In a nutshell, a debugger is a program written to aid in testing and debugging another computer program. For those unfamiliar, debugging is the process of finding and removing defects (bugs) in a computer program and can be accomplished in a variety of ways including examining log files for oddities, monitoring the memory of an application or system, profiling for performance bottlenecks, utilizing test code to discover inconsistencies, and using interactive debugging to examine the running state of your program.

Debuggers come in quite a few different packages and, most likely, you have encountered at least one. Several more well-known debuggers include GDB, commonly used to interactively debug C and C++ programs from the command line, and the Eclipse Debug Project, which is part of the Eclipse IDE and available for a variety of languages although most-specifically targeted towards Java. If you have done any web-based frontend development, you have probably used your browser's debugger such as Google Chrome's DevTools or Mozilla Firefox's Debugger.

While debuggers themselves are an old concept, the latest debuggers are continuing to demonstrate new, clever ideas to make the everyday programmer's life a little easier. If interested in modern developments in the debugger space, check out Elm's time traveling debugger or Scala IDE's new async debugger.


A debugger can offer a wide range of features to aid the programmer in discovering and fixing defects. The most common features found in modern debuggers are the ability to step through a program, stop (break) a program to examine its state, and track the values of variables in a program.

Running a program step-by-step

This involves running a target program one "line" at a time, breaking at each next "line" to observe the state of the program such as the values of variables in the stack frame. While line-by-line stepping is one of the most common and useful forms of stepping through your program, some debuggers offer other step increments such as frame-by-frame.

Newer debuggers like Scala IDE's async debugger offer the ability to step through your program even when the code execution path is not straightforward (in this case when running distributed, message-driven programs using the Akka toolkit and runtime).

Breaking a program to examine its state

This involves setting what is known as a breakpoint, a marker in a program indicating that you want to stop execution temporarily, to be able to examine the running state of a program. Often, breakpoints are placed on specific source lines of a program indicating that the developer wishes to stop the program when executing a specific line to examine the current state. Earlier breakpoints would be placed on specific instructions (assembly).

Other forms of breakpoints can also exist including setting a conditional breakpoint that triggers when reading/writing memory (watchpoints) or the evaluation of an expression yields true.

Watching variables in a program

When debugging a program, a common scenario is that a developer wants to know when a variable's value is accessed or changed. This typically involves the temporary suspension of the program allowing the developer to inspect the current stack frame to understand what led to the access/modification of the variable. Furthermore, more restrictions might be placed on the modification of a variable such that a developer is only notified when a variable is changed to a certain value, allowing more accurate understanding of specific problems such as when a variable unexpectedly becomes null.

Evaluating expressions

A useful feature in many debuggers is the ability to evaluate an expression (snippet of code) in the target program while it is suspended due to a breakpoint. This allows developers to quickly test functions and perform modifications to the state of the program using the same language as the program itself.

More about Eclipse and how it relates to this project

The Eclipse Debug Project offers two subprojects for debugging with the Eclipse tooling: platform debug and JDT debug. Platform debug focuses on defining language-independent debugging capabilities including defining breakpoints (see features for more info), launching programs, and notification of events from target programs. JDT debug is more specific as it implements the Java debugging support to launch or attach to a JVM, evaluate expressions in the context of a stack frame, and dynamically reload classes into the target JVM.

The underlying API driving Eclipse's debugging framework for Java (and other JVM-based languages) is the Java Platform Debugger Architecture (JPDA). The JDPA is a collection of APIs to debug Java code and includes the Java Debug Wire Protocol (JDWP) defining communication between the debugger and target JVMs, the Java Virtual Machine Tools Interface (JVMTI) providing a native interface (C++) to inspect state and control execution of programs running in a JVM, and the Java Debugger Interface (JDI) that defines a high-level API in Java to debug remote JVMs.

The Scala debugger API is built on top of the JDI, wrapping existing JDI functionality in Scala-friendly wrappers and providing higher-level abstractions to aid Scala developers in writing debugger tooling and utilize debugger functionality in their own applications.