Java manual deallocation

If the finalizer raises an exception, the object remains on the heap and the program calling the delete instruction continues executing. The instruction removes just one object from the heap. If it is known that objects referenced by the deleted object are not referenced by any other objects, those can also be freed. As the delete instruction runs the finalizer, the finalizer can be used equivalently to the destructor of non-garbage collected languages and the finalizer can contain explicit deallocation instructions for the objects referenced by the the explicitly deleted object.

It is important to mention that the deletion may cause inconsistency in the heap. If the deleted object is still pointed to by a reference variable, the JVM behaviour will become unpredictable and JVM crash can occur. Bytecode safety is a very attractive feature of Java therefore it must be guaranteed that the delete instruction is used only by trusted code.

Java code shipped with the device e. Before any Java code is installed on the system, a simple filter can detect the presence of delete instructions and reject code that is not privileged enough to contain that instruction.

Alternatively, this mechanism could be placed into Java classloaders; this solution would yield simpler deployment architecture and increased security no way to tamper with downloaded code that already passed the delete filter but slower classloading. Explicit deallocation is not proposed to be used directly from Java programs or from any other pro- gramming language compiled to Java bytecode.

Instead, an algorithm is pro- posed that takes Java bytecode and places the delete instructions and necessary support code at appropriate locations. The algorithm presented here is in many way similar to other dataflow algo- rithms presented e.

If the dataflow analyser cannot find such a region, the object is said to be escaped. Escaped objects can really be long-living objects or can be objects where the dataflow analyser could not figure out the lifetime of the object.

In the presence of asynchronous mech- anisms e. Garbage collection is therefore not eliminated, but the load on the garbage collector can be reduced. This means less garbage collection overhead which yields better performance. Our dataflow algorithm is based on the heuristic observation that a lot of objects with limited lifetime are created during the execution of an average Java program and this limited lifetime is not necessarily short.

As composite data types are always allocated on the heap in Java and Java class library itself is nicely object-oriented the standard class library itself creates quite a lot of objects with limited lifetime the assumption was that significant gain could be realized if these objects were eliminated by explicit deallocation.

Based on the assumption that there exists significant amount of objects with limited lifetime which are used for temporary data storage, the following simplifications were made. If an object is assigned to a global variable, it is considered escaped.

Summary about a method is able to describe only if the object passed to the method as invocation parameter may or may not escape. Escape information about the return value is also available, in this case the escape status means whether there is a possibility that the object to escapes e. The summary is also able to express which, if any, invocation parameters may be returned by the method. Beside simplifications, the dataflow algorithm extends previous algorithms with the notion of stack deallocation.

The term is used for explicitly deallocating an object whose reference is never saved in any variable local or global. Finally the StringBuffer can be garbage- collected.

The StringBuffer reference exists solely on the operand stack and is never saved to any variable. The difference between stack deallocation and deallocation of objects whose reference was stored in local variables exists at bytecode level. If the escape analyser uses a data abstraction other than bytecode e. As it is pointed out later in this section, escape analysis in the presence of a code using conditional branches heavily is very time-consuming operation.

This makes off-line escape analysis attractive. For the purpose of this document the following terms are defined. Definition 1. Global variables are instance or class fields of objects.

Definition 2. Object representation is a representation of an object allocation site. Every object generated at this site is represented by the same object repre- sentation. Object representation describes the allocation site, escape status and the references that objects generated at this site have. S S S Definition 3. The allocation site of an untracked object reference is unknown. Example reference graph Figure 1 illustrates an example reference graph. S Definition 4.

Definition 5. Definition 6. The unification is done is such a way that — every node present in RG1 and RG2 is also present in RGo — every edge pointing to any node is also present in RGo. If the union creates more than 1 outbound edges for a LV node, an intermediate OR node will be created, the LV node points to this intermediate node and the intermediate node points to the nodes that would have been pointed by the LV node after the union.

Definition 7. Summary of a method M captures the escape behaviour of the method regarding its parameters and return value. The unit of processing in our dataflow algorithm is the method. The algo- rithm iterates over the instructions of the method following every path of the CFG and simulates the effects of each instruction on the representation graph and on the operand stack. The methods are visited according to the CFG - methods at the end of the call chain are analysed first.

The exact process is the following. Our implementation allows one reference variable to point to only one node. Union of reference graphs. Each input parameter to the method will be assigned an object representation. The original escape status of these object representations are set according to the method summary.

This operand stack is used by the dataflow analyser to simulate the real run-time JVM operand stack. This means that the element of the operand stack will point to the same object representation as the local variable provided that the edge was not of Eur or Enr type and the object representation will have an additional inbound edge.

If there is none, explicit deallocation site is tentatively placed for nor at this in- struction and the deallocation tuple is added to DSM.

The deallocation site is marked as stack deallocation site. The deallocation site is marked as local variable deallocation site. If it is possible to reach the instruction so that no deallocation site is created or the deallocation site is created with different type or parameters than the existing site at that location then no deallocation site will be created there.

If the summary marks parameter or return value with escaping status, the object representation of that parameter or return value will also have escaping status. The analyser does an exhaustive traversal of all possible contexts that an instruction is reachable from. Figure 3 illustrates this effect. If the analyser finds that the branch target instruction was not yet analysed from every possible context, one copy of the state of the analyser branch target location, data stack and current representation graph is pushed to the analysis stack for that branch target.

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What is the destructor in Java? Advantages of Destructor It releases the resources occupied by the object. No explicit call is required, it is automatically invoked at the end of the program execution. Marco13 Marco13 Sign up or log in Sign up using Google. Sign up using Facebook.

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