Java 内存分析 MAT 工具

Eclipse Memory Analyzer (MAT)

This article describes the usage of the Eclipse Memory Analyzer (MAT) to identify memory leaks.

This article is based on Eclipse 4.5.

1. Memory handling in Java

Java handles its memory in two areas. The heap and the stack. We will start with a short overview of memory in general on a computer. Then the Java heap and stack is explained.

1.1. Native Memory（）

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 than 4 GB of memory.

Of course with a 64-bit OS this 4GB limitation does not exist anymore.

1.5. Escape analysis

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 object 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 than 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.

1.6. Memory leaks

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 Eclipse

2.1. Using heap dumps to get a snapshot of the memory of an application

A heap dump is a snapshot of the complete Java object graph on a Java application 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.

2.2. The Eclipse Memory Analyser (MAT) tooling

The Eclipse Memory Analyser Tooling (MAT) is a set of plug-ins for the Eclipse IDE which provides tools to analyze heap dumps from Java application and to identify memory problems in the application. This helps the developer to find memory leaks and high memory consumption issues.

It visualizes the references to objects based on Java heap dumps and provides tools to identify potential memory leaks.

3. Analyzing Android heap dumps with Eclipse

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 use the Dump HPROF file button in the DDMS Perspective.

The Android heap dump format is similar to the Java heap dump format but not exactly the same. Eclipse MAT can work directly with the Android heap dump format.