Unit testing is now an integral part of the development process for most teams; frameworks like JUnit have made testing painless enough that even if we don't like it, we're willing to write some tests for some of our code. But unit tests operate at a low level; they can only test a single piece of code, and test code generally exhibits a low level of reusability -- the tests we wrote yesterday for component A are unlikely to be much use for testing component B, except maybe as example code.

A typical unit test scenario

When you find a bug, what is the first thing you do? You could just fix it, but that might not be the most efficient approach in the long run. In most development shops, the process looks something like the following:

• Write a test case for the bug
• Make sure the test case fails before touching the code
• Fix the bug
• Make sure the test case now passes
• Make sure the rest of the test suite still passes
• Check the fix and the test case into version control
• Document the fix in the bug tracking system

While this approach is a lot more work in the short term than just fixing the bug, it offers greater value: you have more confidence that the bug is fixed because you have tested it, and you have more confidence that the bug will not come back because the test case is part of your regression test suite. Between the version control system and the bug tracking system, you also have a record that describes what the bug was and how it was fixed -- useful information that may benefit others.

If you're feeling ambitious, you might think about how the bug came about and look for other places where the same mistake was made. Then, if you find the same mistake elsewhere, you might check in tests and fixes for those bugs as well. But the fundamental weakness of unit tests as a quality management tool is that each test case can only test one piece of code. Because test cases must be specifically designed for each component and for each potential failure mode, writing enough unit tests to test a large product can be extremely time-consuming and expensive.

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The economics of QA

Tests are an essential quality management tool, but we all know that having even a great set of test cases is not enough to find all the bugs in a complex piece of software. In fact, "finding all the bugs" for any nontrivial program is likely to be an impossible goal. It is estimated that NASA employs 20 testers for each developer -- far more than any commercial entity could afford to allocate to QA -- and still there are software defects. The goal of the QA process, then, should not be to find all the bugs, because that's impossible. Instead, it should be to raise confidence that the code works, to the greatest degree possible given the available resources.

Running an efficient QA operation involves budgeting available resources among the available QA methodologies so as to maximize confidence. A test suite with good coverage raises our confidence that the code works, as does a thorough code review. Doing both is better than doing just one or the other because each finds errors the other is likely to miss. Both are also subject to diminishing returns, so a QA plan that involves X dollars worth of testing and Y dollars worth of code review is likely to be more effective than one that involves X+Y of either.

Adding static analysis

Static analysis is the process of analyzing code without running it, much like what we do in our heads during a code review or what IDEs do when they flag questionable constructs. Static analysis is a sensible technique to add to the QA mix because it is yet another technique that is good at finding errors that other approaches (like testing and code review) can miss. Static analysis is also relatively cheap; unlike unit tests, which you must write anew for each class you want to test, you can run static analysis tools on any body of code.

FindBugs is an open source static analysis tool that contains bug pattern detectors for many common bug patterns. Perhaps surprisingly, it often can find "dumb" bugs in even well-tested software -- bugs that unit tests and expert code reviews are likely to miss. FindBugs also allows you to write new bug pattern detectors and package them as plugins so if the standard set of detectors doesn't do what you need, you can easily write your own. It is this extensibility that makes FindBugs such a powerful tool for quality management because as you discover new types of mistakes, you can write detectors for them and search for them in your entire code base.

The major cost of static analysis is analyzing the output and determining if reported items are actual bugs or false alarms. Part of writing a good analysis tool or bug pattern detector is managing the false alarm rate; the detectors in the core FindBugs package have been tuned with the goal of emitting no more than 50 percent false alarms so that analyzing the output is not excessively painful. (Contrast this threshold with lint-like tools for C, which often emit so many false alarms that they are prohibitively time-consuming to use.)

Taking it up a level

The methodology described earlier for fixing a bug -- write the test case first, then fix the bug, then check in both the fix and the test case -- reflects the desire not only to fix the bug but to improve confidence that it has been fixed and to retain the knowledge of how and when it was fixed. This approach is more work than just fixing the bug, but it gives us more confidence that our code will continue to work even with ongoing modification by multiple developers. But writing test cases only for bugs that have been discovered is a reactive approach. We would like to analyze our code to the extent possible for compliance with good practices before it fails.

Listing 1 illustrates a common bug with the use of the BigDecimal class. BigDecimal is immutable, so the arithmetic methods such as add() return a new BigDecimal as their result rather than modifying the object on which they are invoked. The code in Listing 1 clearly is supposed to add conditionally the shipping cost to the total order price, but in fact unconditionally adds nothing because the return value of add() is discarded:

public class ShoppingCart { 
    private BigDecimal totalCost;

    private boolean qualifiesForFreeShipping() { ... }
    private BigDecimal getShippingCost() { ... }

    public void checkout() {
        ...
        if (!qualifiesForFreeShipping())
            totalCost.add(getShippingCost()); //WRONG!
    }
}

The mistake in Listing 1 -- forgetting that an object is immutable and thus mistaking a factory method for a mutator method -- is a common one. If you were to find such a mistake in your code, you might realize that there is a good chance that the same mistake is made more than once, as it stems from a misunderstanding of how a particular library class works. On discovering this bug, a responsible developer might search the entire code base for calls to BigDecimal.add(), subtract(), and so on, looking for other instances of ignoring the return value.

This strategy is a good first step, but we can do better. It is easy to identify the bug pattern here -- ignoring the result of a value-bearing method on an immutable object. Once you identify the pattern, it is a relatively simple matter to build a detector that identifies this pattern. (FindBugs has such a detector in the core detector set.) Not only can this technique apply to BigDecimal, but also to other immutable classes such as BigInteger, String, or Color.

Taking the time to create a bug detector for a bug pattern such as this one can pay big dividends. Not only can you audit your entire project for this bug with less work and higher confidence than you can by hand, but you can apply the same detector to other projects, present and future. Rather than addressing bugs on an instance-by-instance basis, you've created a defense against that type of bug ever coming back, wherever it might pop up.

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An example bug detector

To illustrate the process of writing a detector for FindBugs, let's write a simple detector that finds calls to System.gc(). (Download the source code for this example detector code.) While calling System.gc() is not necessarily a bug, in practice it usually causes more problems than it solves. Having an errant call to System.gc() buried in a library can dramatically impair the performance of a program that uses that library, and the developers may well be left scratching their heads, wondering why performance is so bad.

The first step in writing a bug detector is to identify the bug pattern being detected. In this case, the pattern is simple: calling System.gc(). To write a detector that recognizes this pattern in bytecode, we need to know what the bytecode corresponding to the bug pattern looks like. The easiest way to learn the answer is to write a small program that has the bug, compile it, and disassemble the .class file with javap -c. Listing 2 shows a class that exhibits the bug:

public class BadClass { 
    public void doBadStuff() {
        System.gc();
    }
}

Listing 3 shows the output of javap -c when run on the sample class:

public void doBadStuff();
  Code:
   0:	invokestatic	#2; //Method java/lang/System.gc:()V
   3:	return

We can quickly see that calling a static method is done with the invokestatic JVM instruction, and the operand of invokestatic is the gc:()V method of class java/lang/system. The method signatures and type names in the bytecode look a little different than they do in the source code, but it doesn't take long to get used to the encoding used by the bytecode.

With this example of our bug pattern, writing a FindBugs detector is fairly straightforward. Listing 4 shows a detector that extends the BytecodeScanningDetector base class and overrides the sawOpcode() method. When it sees an invokestatic instruction, it checks the class and name of the method being invoked and if it is System.gc(), it reports a bug instance.

public class CallSystemGC extends BytecodeScanningDetector {
    private BugReporter bugReporter;

    public CallSystemGC(BugReporter bugReporter) {
        this.bugReporter = bugReporter;
    }
    
    public void sawOpcode(int seen) {
        if (seen == INVOKESTATIC) {
            if (getClassConstantOperand().equals("java/lang/System")
                    && getNameConstantOperand().equals("gc")) {
                bugReporter.reportBug(new BugInstance("SYSTEM_GC", NORMAL_PRIORITY)
                        .addClassAndMethod(this)
                        .addSourceLine(this));
            }
        }
    }
}

Packaging detectors into plugins

The last step required in creating a new bug detector is to package it as a plugin. A FindBugs plugin contains one or more bug detectors, a deployment descriptor, and a resource file, packaged into a JAR file and placed in the plugins directory of your FindBugs installation. The deployment descriptor, called findbugs.xml, defines the known bug detectors and the errors it might report. The resource file, called messages.xml (or messages_xx.xml, for localized versions), defines language-specific strings that will be used by the FindBugs GUI to describe the bugs reported. The deployment descriptor and resource file for our example bug detector are shown in Listing 5 and Listing 6. Multiple localized versions of the resource file may be included in the plugin JAR; the deployment descriptor and resource files are placed in the top level directory of the plugin JAR.

<FindbugsPlugin xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   				xsi:noNamespaceSchemaLocation="findbugsplugin.xsd"
				pluginid="com.briangoetz.findbugs.plugin"
                defaultenabled="true"
                provider="Brian Goetz"
                website="http://www.briangoetz.com">

  <Detector class="com.briangoetz.findbugs.plugin.CallSystemGC"
	        speed="fast"
	        reports="SYSTEM_GC" />

  <BugPattern abbrev="GC" type="SYSTEM_GC" category="PERFORMANCE" />
</FindbugsPlugin>
<

<MessageCollection xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                  xsi:noNamespaceSchemaLocation="messagecollection.xsd">

  <Plugin>
    <ShortDescription>Brian's plugin</ShortDescription>
    <Details></Details>
  </Plugin>

  <Detector class="com.briangoetz.findbugs.plugin.CallSystemGC">
    <Details>
<![CDATA[
Finds calls to System.gc().
]]>
    </Details>
  </Detector>

  <BugPattern type="SYSTEM_GC">
    <ShortDescription>Method calls System.gc()</ShortDescription>
    <LongDescription>Call to System.gc() method in {1}</LongDescription>
    <Details>
<![CDATA[
Library code should not call System.gc()
]]>
    </Details>
  </BugPattern>

  <BugCode abbrev="GC" >Garbage collection</BugCode>
</MessageCollection>

Building and packaging our plugin and running it against the JDK 1.4.2 class libraries provides us with a surprise: several classes in com.sun.imageio -- including JPEGImageReader and JPEGImageWriter -- call System.gc()! This result is yet another benefit of the flexibility of static analysis: Once you have created a bug detector, you may be surprised at where it finds bugs.

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Summary

Static analysis and custom bug detectors can be a very cost-effective way to improve software quality. By creating a detector for a known bug pattern, we can search for that bug pattern not only in the current code base for a specific project, but in any project, current or future. The extra effort to create a bug detector is more than made up for by the quality dividends it pays in the future.

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Download

Information about download methods

Resources

Learn

• Java theory and practice: "Kill bugs dead" (developerWorks, Brian Goetz, June 2004): This earlier installment of Java theory and practice introduced the FindBugs tool and described some of the bug patterns it can find.
  
  
• "FindBugs, Part I" (developerWorks, Chris Grindstaff, May 2004): Discusses how to integrate FindBugs into your development methodology.
  
  
• "FindBugs, Part II" (developerWorks, Chris Grindstaff, May 2004): The follow-on article discusses how to use custom bug detectors to enforce project-wide code standards.
  
  
• The Java technology zone: Hundreds of articles about every aspect of Java programming.
  
  

Get products and technologies

• FindBugs: Download FindBugs and try it out on your code.
  
  

About the author

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Table of contents

• A typical unit test scenario
• The economics of QA
• An example bug detector
• Summary
• Download
• Resources
• About the author
• Comments

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