IOS libsystem_c.dylib strdup memory leak NSZombie not working

Here's a good way to create a true memory leak (objects inaccessible by running code but still stored in memory) in pure Java:

1. The application creates a long-running thread (or use a thread pool to leak even faster).
2. The thread loads a class via an (optionally custom) ClassLoader.
3. The class allocates a large chunk of memory (e.g. new byte[1000000]), stores a strong reference to it in a static field, and then stores a reference to itself in a ThreadLocal. Allocating the extra memory is optional (leaking the class instance is enough), but it will make the leak work that much faster.
4. The application clears all references to the custom class or the ClassLoader it was loaded from.
5. Repeat.

Due to the way ThreadLocal is implemented in Oracle's JDK, this creates a memory leak:

• Each Thread has a private field threadLocals, which actually stores the thread-local values.
• Each key in this map is a weak reference to a ThreadLocal object, so after that ThreadLocal object is garbage-collected, its entry is removed from the map.
• But each value is a strong reference, so when a value (directly or indirectly) points to the ThreadLocal object that is its key, that object will neither be garbage-collected nor removed from the map as long as the thread lives.

In this example, the chain of strong references looks like this:

Thread object → threadLocals map → instance of example class → example class → static ThreadLocal field → ThreadLocal object.

(The ClassLoader doesn't really play a role in creating the leak, it just makes the leak worse because of this additional reference chain: example class → ClassLoader → all the classes it has loaded. It was even worse in many JVM implementations, especially prior to Java 7, because classes and ClassLoaders were allocated straight into permgen and were never garbage-collected at all.)

A variation on this pattern is why application containers (like Tomcat) can leak memory like a sieve if you frequently redeploy applications which happen to use ThreadLocals that in some way point back to themselves. This can happen for a number of subtle reasons and is often hard to debug and/or fix.

Update: Since lots of people keep asking for it, here's some example code that shows this behavior in action.

Well, GUIDs are easy and look like the best fit. They're tempting to frontend programmers, since they don't have to deal with the database.

On the other hand, when looking at the potential drawbacks they have when used without thinking about database issues too much, I would warn against them as much as possible.

The question really is: do you really need to know the ID of the entity before it's stored in the databsae? REALLY? WHY?

If you do decide to go with GUIDs in the end, and if you're using SQL Server as your database backend (I don't know enough about other RDBMS to make an informed suggestion), I would strongly recommend you make absolutely sure that the GUIDs are not being used as the clustering key on the tables. This will kill your performance - no doubt.

If you do use GUIDs as your primary key, make sure to use something else, some other column that wrecks less havoc on your database, as your clustering key instead - a INT IDENTITY would be my first choice.

Check out these articles by Kimberly Tripp on why a GUID is definitely not a good idea as a clustering key in a SQL Server database - she's the ultimate guru when it comes to indexing and performance issues with indexing and she can makes those points much better than I ever could:

• GUIDs as PRIMARY KEYs and/or the clustering key
• The Clustered Index Debate Continues...
• Ever-increasing clustering key - the Clustered Index Debate..........again!

Marc