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About Gerard Davison

Persistent breakpoints every java developer should have

When a developer is working in Java there are a few failure cases you always want to know about even if you were trying to debug something else. Here is a list of the persistent breakpoints that should be enabled in every IDE. (IMHO of course)

Yes in theory you should be able to just get this from a good logging implementation; but often these problems are deep down in somebody else’s library caused by changes that are beyond your control….. or just hidden in a large number of currently executing threads. Java these days doesn’t have that much of an overhead to be running in debug mode, generally I never run anything I am working on without a debugger attached as HotSwap is such a productivity boost.
I would be interested to heard of other people’s must have breakpoints. They should be failures that should never come up in the normal running of your application that you would want to know about right away.

Deadlock detection

The VM is getting better and better at recognising deadlocks from low level java contracts, you debugger should have an option to break on deadlock. You should make sure it is turned on by default.

On java.lang.ExceptionInInitializer

This one can be a right pain to track down because further down the line it can become a ClassNotFoundException and often if it is more than a few levels down there is no obvious reason why the indirectly referencing class has failed to load.

Often caused by people initialising public/final/static variables with methods/constructors that throw RuntimeExceptions. If you are not sure use a static{} block with proper error handling, never do something in there that depends on something external to the current jar file.

On java.lang.NoClassDefFoundError

Always a popular when working in OSGi, normally you have forgotten a dependant jar file and you see this type of failure. Interestingly you sometimes only get a cause for the first time this exception is thrown for a particular class. So if you have a breakpoint then you can more quickly track this down.

I have found that sometimes that the breakpoint is not hit when behind some reflective code. In that case I often have breakpoints in the constructor of this class just to make sure.

On java.lang.NoSuchMethodError

You will of course generally see this if you haven’t built locally properly or you are using miss-matched versions of libraries. You always want to know about this as soon as it happens.

On java.lang.LinkageError

This normally only turns up when things really go bad, for example when you have two versions of the same class hanging about even though they are from the same jar file. (Gotta love multiple class loaders) Be thankful these don’t turn up very often, normally timed for a day when I have a really bad headache already.

On java.lang.OutOfMemoryError / java.lang.StackOverflowException

If you are very lucky the VM will breakpoint for these; but unfortunately as a lot of time this will happen in native code you will just have to deal with the fallout.

Getting the full stack for the latter is a bit of a pain; but not impossible. See this older post on getting the full stack trace.

On java.lang.AssertionError

A surprising number of people use assertions and then get upset when you complain about there exceptions when debugging or running tests. (You are doing this with -ea aren’t you?)

The downside is that you want to turn this off when debugging certain testing frameworks, I am looking at you JUnit, as it is not entirely helpful to hit a breakpoint when you just want to run all of your tests.

Finally every now and again I will run across the following code, which causes an exception just to test if assertions are enabled.

boolean assertionsEnabled = false;
    { assert false; }
    catch (AssertionException ae)
    { assertionsEnabled = true }

Update: It has been pointed out by a helpful colleague at that I should have provided some alternative suggestions as to what this code should be replaced with. The first option would be to call Class.desiredAssertionStatus but if you want to be totally sure you can use the following code that uses assignment to capture the assertion status without throwing an exception.

boolean assertionsEnabled = false;

    assert isAssertionsEnabled = true;


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