1. Memory handling in Java

Native memory is the memory which is available to a process, e.g. the Java process. Native memory is controlled by the operating system (OS) and based on physical memory and other physical devices, e.g. disks, flash memory, etc.

The processor (CPU) of the computer computes the instructions to execute and stores its computation results into registers. These registers are fast memory elements which stores the result of the CPU. The processor can access the normal memory over the memory bus. A amount of memory a CPU can access is based on the size of the physical address which the CPU uses to identify physical memory. A 16-bit address can access 2^16 (=65.536) memory locations. A 32-bit address can access 2^32 (=4.294.967.296) memory locations. If each memory area consists of 8 bytes then a 16-bit system can access 64KB of memory and the 32-bit system can access 4GB of memory.

An operating system (OS) normally uses virtual memory to map the physical memory to memory which each process can see. The OS assigns then memory to each process in a virtual memory space for this process and maps access to this virtual memory to the real physical memory.

Current 32-bit systems uses an extension (Physical Address Extension (PAE)) which extends the physical space to 36-bits of the operation system. This allows the OS to access 64GB. The OS uses then virtual memory to allow the individual process 4 GB of memory. Even with PAE enabled a process can not access more then 4 GB of memory.

Of course with a 64-bit OS this 4GB limitation does not exists any more.

Java manages the memory for use. New objects created and placed in the heap. Once your application have no reference anymore to an objects the Java garbage collector is allowed to delete this object and remove the memory so that your application can use this memory again.

In the heap the Java Virtual Machine (JVM) stores all objects created by the Java application, e.g. by using the "new" operator. The Java garbage collector (gc) can logically separate the heap into different areas, so that the gc can faster identify objects which can get removed

The memory for new objects is allocated on the heap at run time. Instance variables live inside the object in which they are declared.

Stack is where the method invocations and the local variables are stored. If a method is called then its stack frame is put onto the top of the call stack. The stack frame holds the state of the method including which line of code is executing and the values of all local variables. The method at the top of the stack is always the current running method for that stack. Threads have their own call stack.

As stated earlier Java objects are created and stored in the heap. The programming language does not offer the possibility to let the programmer decide if an objects should be generated in the stack. But in certain cases it would be desirable to allocate an object on the stack, as the memory allocation on the stack is cheaper then the memory allocation in the heap, deallocation on the stack is free and the stack is efficiently managed by the runtime.

The JVM uses therefore internally escape analysis to check if an object is used only with a thread or method. If the JVM identify this it may decide to create the object on the stack, increasing performance of the Java program.

The garbage collector of the JVM releases Java objects from memory as long as no other object refers to this object. If other objects still hold references to these objects, then the garbage collector of the JVM cannot release them.

2. Analyzing memory leaks with with Eclipse

A Java heap dump is a snapshot of the complete Java object graph at a certain point in time. It is stored in a binary format called HPROF.

It includes all objects, fields, primitive types and object references.

The Eclipse MAT tooling is a set of plug-ins which visualize the references to objects based on Java heap dumps and provides tools to identify potential memory leaks.

It is possible to instruct the JVM to create automatically a heap dump in case that it runs out of memory, i.e. in case of a OutOfMemoryError error. To instruct the JVM to create a heap dump in such a situation, start your Java application with the -XX:+HeapDumpOnOutOfMemoryError option.

Android allows to create heap dumps of an application's heap. This heap dump is stored in a binary format called HPROF. To create a heap dump us the Dump HPROF file button in the DDMS Perspective.

The Dalvik format is similar to the Java heap dump format. To convert the Dalvik heap dump to the Java heap format you can use the hprof-conv source target command.

After a new heap dump with the .hprof ending has been created, you can open it via a double-click in Eclipse. You may need to refresh your project (F5 on the project). Double-click the file and select the Leak Suspects Report.

The overview page allows you to start the analysis of the heap dump. The dominator tree gives quickly an overview of the used objects.

3. Installation

4. Example

Create the Java project called com.vogella.mat.first and the com.vogella.mat.first package. Create the following class.

package com.vogella.mat.first; import java.util.ArrayList; import java.util.List; public class Main {

In Eclipse add the -XX:+HeapDumpOnOutOfMemoryError to the runtime configuration.

Run the project. It crashes and write an heap dump.

5. jconsole

6. Thank you

7. Questions and Discussion

8. Links and Literature

 Android and Eclipse Training from the vogella team

 Introduction to Android Programming

 Program in Java, compile to JavaScript and HTML

 Create native applications in Java

 Test your application

 Put all your files in a distributed version control system