Possible heap pollution via varargs parameter

Question

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

User · Accepted Answer

Heap pollution is a technical term  It refers to references which have a type that is not a supertype of the object they point to   List lt A gt  listOfAs   new ArrayList lt  gt     List lt B gt  listOfBs    List lt B gt   Object listOfAs     points to a list of As   This can lead to  unexplainable  ClassCastExceptions      if the heap never gets polluted  this should never throw a CCE B b   listOfBs get 0       SafeVarargs does not prevent this at all  However  there are methods which provably will not pollute the heap  the compiler just can t prove it  Previously  callers of such APIs would get annoying warnings that were completely pointless but had to be suppressed at every call site  Now the API author can suppress it once at the declaration site   However  if the method actually is not safe  users will no longer be warned

User · Answer

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 lt Type1 lt Type2 gt  gt   input instead of varargs  Type1 lt Type2 gt       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

User · Answer

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

User · Answer

When you declare  public static  lt T gt  void foo List lt T gt     bar  the compiler converts it to  public static  lt T gt  void foo List lt T gt    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

User · Answer

The reason is because varargs give the option of being called with a non-parametrized object array  So if your type was List  lt  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 lt A gt     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

User · Answer

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

[java] Possible heap pollution via varargs parameter

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