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https://issues.apache.org/jira/browse/GROOVY-10657?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17554129#comment-17554129
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Eric Milles commented on GROOVY-10657:
--------------------------------------
As we process the assignment expression `employees["1"] = "Bob"` we have to
deal with the RHS to understand what it is (see below). Just after this, we
need to stash the top operand into a temp var so that we can push `employees`
and `"1"` onto the stack. Then the RHS is pushed back. At this point, the
temp could be discarded I suppose.
>From {{BinaryExpressionHelper}}:
{code:java}
public void evaluateEqual(final BinaryExpression expression, final boolean
defineVariable) {
AsmClassGenerator acg = controller.getAcg();
CompileStack compileStack = controller.getCompileStack();
OperandStack operandStack = controller.getOperandStack();
Expression leftExpression = expression.getLeftExpression();
Expression rightExpression = expression.getRightExpression();
boolean directAssignment = defineVariable && !(leftExpression
instanceof TupleExpression);
if (directAssignment && rightExpression instanceof EmptyExpression) {
VariableExpression ve = (VariableExpression) leftExpression;
BytecodeVariable var = compileStack.defineVariable(ve,
controller.getTypeChooser().resolveType(ve, controller.getClassNode()), false);
operandStack.loadOrStoreVariable(var, false);
return;
}
// evaluate the RHS and store the result
// TODO: LHS has not been visited, it could be a variable in a closure
and type chooser is not aware.
ClassNode lhsType =
controller.getTypeChooser().resolveType(leftExpression,
controller.getClassNode());
if (rightExpression instanceof ListExpression && lhsType.isArray()) {
ListExpression list = (ListExpression) rightExpression;
ArrayExpression array = new
ArrayExpression(lhsType.getComponentType(), list.getExpressions());
array.setSourcePosition(list);
array.visit(acg);
} else if (rightExpression instanceof EmptyExpression) {
loadInitValue(leftExpression.getType()); // TODO: lhsType?
} else {
rightExpression.visit(acg);
}
ClassNode rhsType = operandStack.getTopOperand();
int rhsValueId;
{code}
> Generated code for overloaded assignment operator prevents garbage collection
> -----------------------------------------------------------------------------
>
> Key: GROOVY-10657
> URL: https://issues.apache.org/jira/browse/GROOVY-10657
> Project: Groovy
> Issue Type: Bug
> Components: Compiler
> Affects Versions: 4.0.3
> Reporter: Jonathan Baxter
> Priority: Major
>
> The groovy code
> {code:java}
> obj["lhs"] = "rhs"{code}
> ostensibly converts to
> {code:java}
> obj.putAt("lhs","rhs")
> {code}
> if {{obj}} has the appropriate {{putAt}} method.
> However , in the process of tracking down a memory leak, I discovered that
> assignment is not exactly equivalent to invocation of the {{putAt}} method.
> Specifically, the code generated by the compiler is equivalent to creating an
> additional temporary variable to hold the rhs of the assignment, and then
> invoking the {{putAt}} method with that temporary variable as its second
> argument:
> {code:java}
> String string = "rhs"
> obj.putAt("lhs",string){code}
> The problem with this is that since {{"rhs"}} now has a live reference
> ({{{}string{}}}), it is not eligible for garbage collection.
> In my particular use case, {{"rhs"}} is a very large object that I would like
> to be automatically garbage collected after the assignment is executed, but
> before the script has finished. But that means I have to use the underlying
> {{putAt}} method rather than the assignment operator, which rather negates
> the benefits of overloading the assignment operator.
> One potential fix would be to simply null out any temporary variables
> constructed by the compiler, so that
> {code:java}
> obj["lhs"] = "rhs"
> {code}
> becomes
> {code:java}
> String string = "rhs"
> obj.putAt("lhs",string)
> string = null <----- "rhs" is now eligible for garbage-collection{code}
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