[java] Possible heap pollution via varargs parameter

I understand this occurs with Java 7 when using varargs with a generic type;

But my question is..

What exactly does Eclipse mean when it says "its use could potentially pollute the heap?"

And

How does the new @SafeVarargs annotation prevent this?

When you use varargs, it can result in the creation of an Object[] to hold the arguments.

Due to escape analysis, the JIT can optimise away this array creation. (One of the few times I have found it does so) Its not guaranteed to be optimised away, but I wouldn't worry about it unless you see its an issue in your memory profiler.

AFAIK @SafeVarargs suppresses a warning by the compiler and doesn't change how the JIT behaves.

@SafeVarargs does not prevent it from happening, however it mandates that the compiler is stricter when compiling code that uses it.

http://docs.oracle.com/javase/7/docs/api/java/lang/SafeVarargs.html explains this in futher detail.

Heap pollution is when you get a ClassCastException when doing an operation on a generic interface and it contains another type than declared.

When you declare

public static <T> void foo(List<T>... bar) the compiler converts it to

public static <T> void foo(List<T>[] bar) then to

public static void foo(List[] bar)

The danger then arises that you'll mistakenly assign incorrect values into the list and the compiler will not trigger any error. For example, if T is a String then the following code will compile without error but will fail at runtime:

// First, strip away the array type (arrays allow this kind of upcasting)
Object[] objectArray = bar;

// Next, insert an element with an incorrect type into the array
objectArray[0] = Arrays.asList(new Integer(42));

// Finally, try accessing the original array. A runtime error will occur
// (ClassCastException due to a casting from Integer to String)
T firstElement = bar[0].get(0);

If you reviewed the method to ensure that it doesn't contain such vulnerabilities then you can annotate it with @SafeVarargs to suppress the warning. For interfaces, use @SuppressWarnings("unchecked").

If you get this error message:

Varargs method could cause heap pollution from non-reifiable varargs parameter

and you are sure that your usage is safe then you should use @SuppressWarnings("varargs") instead. See Is @SafeVarargs an appropriate annotation for this method? and https://stackoverflow.com/a/14252221/14731 for a nice explanation of this second kind of error.

References:

• http://docs.oracle.com/javase/7/docs/technotes/guides/language/non-reifiable-varargs.html
• http://docs.oracle.com/javase/tutorial/java/generics/nonReifiableVarargsType.html#heap_pollution

It's rather safe to add @SafeVarargs annotation to the method when you can control the way it's called (e.g. a private method of a class). You must make sure that only the instances of the declared generic type are passed to the method.

If the method exposed externally as a library, it becomes hard to catch such mistakes. In this case it's best to avoid this annotation and rewrite the solution with a collection type (e.g. Collection<Type1<Type2>>) input instead of varargs (Type1<Type2>...).

As for the naming, the term heap pollution phenomenon is quite misleading in my opinion. In the documentation the actual JVM heap is not event mentioned. There is a question at Software Engineering that contains some interesting thoughts on the naming of this phenomenon.

The reason is because varargs give the option of being called with a non-parametrized object array. So if your type was List < A > ... , it can also be called with List[] non-varargs type.

Here is an example:

public static void testCode(){
    List[] b = new List[1];
    test(b);
}

@SafeVarargs
public static void test(List<A>... a){
}

As you can see List[] b can contain any type of consumer, and yet this code compiles. If you use varargs, then you are fine, but if you use the method definition after type-erasure - void test(List[]) - then the compiler will not check the template parameter types. @SafeVarargs will suppress this warning.