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What is FindBugs?

    FindBugsis an open source static analysis tool that analyzes Java class files

    Developed from Research at University of Maryland, led by Bill Pugh

    Looks for  for programming defects based on  ~ 300 different bug patterns from real bugs

    bug patterns are grouped into categories: correctness, bad practice, performance…

    each pattern assigned a priority: high, medium or low.

    High-Medium priority have low false positive rates: effort was put into trying to ensure that issues reported as high or medium priority correctness issues were issues that had a low false positive rate, and that developers would be interested inexamining all issues reported as high or medium priority correctness issues, even in large code bases.


FindBugslooks for  Bugs  based on real bug Patterns:

     broad and common patterns:

     a read or write on a null pointer


     Methods whose return value should not be ignored

     Also specific bug patterns:

     Every Programming Puzzler

     Eclipse documented bug fixes

     Every chapter inEffective Java

     lots from Worse than failure:



Misconceptions about Bugs:

    Programmers are smart

    Smart people don’t make dumb mistakes


    Smart people make dumb mistakes

    Common errors:

    wrong booleanoperator, forgetting parentheses, etc. Misunderstood class or method !


Who uses Findbugs?

    Developed from Research at University of Maryland

    Google, Ebay, Sun, Wells Fargo…

    Bill Pugh , Professor from University of Maryland, spent a year sabbatical at Google  working Findbugs  into their development process:

    Google runs FindBugs over all Java code

    1800s issues identified, > 600 fixed.

    Ebay found 2 developers reviewing  Findbugs  was 10 times more effective than 2 testers


Some Bug Categories

• Correctness - the code is doing something wrong, you should look at it

• Bad practice - the code violates good practice

     Dodgy Code



• Security defect


Can you find the Bug below ?

publicStringsendMessage (User user, String body, Date time) {

    return sendMessage(user, body, null);


publicStringsendMessage (User user, String body, Date time, List attachments) {

   String xml = buildXML (body, attachments);

   String response = sendMessage(user, xml);

   return response;


This line causes an Infinite recursive loop. This bug is  high priority,  in the correctness category.

returnsendMessage(user, body, null);


Can you find the Bug below ?

publicStringfoundType() {

   return this.foundType();


Should be

publicStringfoundType() {

   return this.foundType;



This is another example of an infinite recursive loop. This bug is found often  in a decorator pattern when you forget to delegate to another method

 Findbugs found 5 infinite recursive loopsin JDK1.6.0-b13. Including this one written by Joshua Bloch

• Smart people make dumb mistakes

• 27 across all versions of JDK, 31 in Google’s Java code

• Embrace and fix your dumb mistakes!


Can you find the Bug?

if(name != null || name.length > 0)


Should be

if(name != null && name.length > 0)

this null pointer bug was found in com.sun.corba


Can you find the Bug?
privatefinal String _lock = "LOCK";

Constant Strings are shared (even private ones) across all other classes loaded by the JVM. This could lead to unexpected deadlocks in conjunction with other code. This bug was found in Jetty.


This null pointer bug was in Eclipse since 3.2:

if(adapters == null && adapters.length == 0)


•example of  statement orbranch that if executed guarantee that a null pointer exception will occur

but in this case adapters is probably never null, that’s why it didn’t get noticed.

     somemistakes don’t matter much, because  the impact of the mistake is minimal.

null pointer exceptions usually get noticed. A harder error to find is that it won’t return if the length is 0.


Can you find the bug?

try{ ... }

catch(IOException e) {

  newSAXException("Server side Exception:" + e);


The Exception is created and dropped rather than thrown, Should be :

  thrownew SAXException("Server side Exception:" + e);


Can you find the Bug?

publicstatic String getNameById(String userId) {

    String str = userId;

    str.replace(' ', '_');

    return str;



Ignores the return value of the replace() method, Should be:

     str= str.replace(' ', '_');


A very common mistake is ignoring the return value on methods whose return value shouldn't be ignored.

• Strings are immutable, so functions like trim() and replace() return a new String.


What does this Print?

Integer one = 1;

Long addressTypeCode = 1L;

if(addressTypeCode.equals(one)) {


} else {

     System.out.println("not equals");



It prints "not equals". According to the contract of equals(), objects of different classes should always compare as unequal

Incomparable equality:

     Using .equals to compare incompatible types

     Using .equals to compare arrays

     only checks if the same array

     Checking to see if a Set<Long> contains anInteger

     never found, even if the same integral value is contained in the map

     Calling get(String)on a Map<Integer,String>

     Returns false , not an error

     These bugs may be hard to find on your own

     Types not always explicit

     May be introduced by refactoring

           For example a Google refactoring changed a method to return byte[ ] rather than String

Bugs that Matter…

You can have mistakes which don't actually cause problems. Bugs that matter depend on the context.  Not every bug found matters, but the effective use of Findbugs can remove bugs cheaper than with other methods

What is the best way to use Findbugs?

You want to find an effective way to use static analysis to improve software quality. 5-10% of mistakes can be found with Findbugs, for some types of bugs it can find all of them.  Testing and running code in production will find the bugs that matter, the bugs that cause problems .  If you run Findbugs on code in production , then a lot of bugs have been fixed, if you run on new code your are more likely to find bugs. The reason to use findbugs is not to fix all your quality problems, but rather for the issues it does find,  finding and fixing with Findbugs is cheaper than finding with testing or causing a problem in production.

     Use Findbugs on new code to remove bugs cheaplybefore bugs are detected using more expensive techniques

     The best time to have a developer  look at a warning is when it is introduced, while the code is fresh in a developer’s head.

Expensive Mistakes are …

     Mistakes that fail silently

     Bugs that silently cause the wrong answer to be computed are the scariest bugs, because they will not be noticed easily .

     Mistakes that cause loss of money when they occur

     Mistakes that are hard to fix

Mistakes that matter ($$)  can be found by testing or eventually in production,  but it is cost effective to find bugs  before testing and production with findbugs.


Runtime exceptions can be your friend… runtime exceptions can point you right where the error is

If a mistake causes a runtime exception, then the mistake will be found and corrected .

Throwing a runtime exception is often a reasonable way to fail safely and report a failure.

Runtime exceptions represent conditions that reflect errors in your program's logic and cannot be reasonably recovered from


Can you find the Bug?

// calculate DR amount by aggregating CR amounts

BigDecimaldrAmount = new BigDecimal(0);

for(JournalEntry je: journalEntries)


     // persist to db


        drAmount, // aggregated amount

        true, // Debit




Ignores return value of BigDecimal.add(), should be


This bug would have cost $ in production, but fixed at Google, within 30 minutes of being reported.


Can you find the Bug?


Public boolean equals(Integer value1){

  returnvalue1== intValue() ;


publicInteger intValue() {




Uses reference equality rather than .equals, should be:

Public boolean equals(Integer value1){

  returnvalue1.equals(intValue() );


For boxed primitives, == and != are computed using pointer equality, but <, <=, >, >= are computed by comparing unboxed primitive values. This can bite you on other classes (e.g., String), but boxed primitives is where people get bit


Concurrency Bugs

     Inconsistent synchronization –

     a lock is held   sometimes when field accessed

     Problems with wait/notify –

     e.g., call to wait() not in loop

     unsafe lazy initialization of static field


Can you find the bug?

synchronized(object) {

  if (<condition does not hold>) {



  //Proceed when condition holds



Should be:

synchronized(object) {

  while (<condition does not hold>) {



  //Proceed when condition holds



Use Java 5!  simplified concurrency , less likely to make these mistakes.


Running FindBugs

     Eclipse plugin


     Run with Hudson build


     Findbugs home page


     Bill Pugh Findbugs Devoxxtalk


     Bill Pugh Oredev talk:



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