However, you need to provide these tools with heap dumps captured in the correct format. This article gives you multiple options to capture heap dumps. This tool is packaged within JDK. If this option is passed, then only live objects in the memory are written into the heap dump file. If this option is not passed, all the objects, even the ones which are ready to be garbage collected are printed in the heap dump file. It will increase the heap dump file size significantly.
It will also make the analysis tedious. When an application experiences java. OutOfMemoryError occurred. Not only that, they also restart the application. Best Practice : Keep this property configured in all the applications at all the times, as you never know when OutOfMemoryError will happen. When you launch this tool, you can see all the Java processes that are running on the local machine.
You can also connect to java process running on a remote machine using this tool. After running this command the heap dump file with extension hprof is created. The option live is used to collect only the live objects that still have a reference in the running code.
With the live option, a full GC is triggered to sweep away unreachable objects and then dump only the live objects. This option is used to capture a heap dump at the point in time when an OutOfMemoryError occurred. This helps to diagnose the problem because we can see what objects were sitting in memory and what percentage of memory they were occupying right at the time of the OutOfMemoryError.
We will use this option for our example since it will give us more insight into the cause of the crash. After running our Java program with these VM arguments, we get this output:. As we can see from the output, the heap dump file with the name: hdump.
It is packaged as part of the JDK. JVisualVM : Usually, analyzing heap dump takes more memory than the actual heap dump size. This could be problematic if we are trying to analyze a heap dump from a large server on a development machine.
JVisualVM provides a live sampling of the Heap memory so it does not eat up the whole memory. Let us understand the basic concepts of Java heap dump analysis with MAT by analyzing the heap dump file we generated earlier. We will first start the Memory Analyzer Tool and open the heap dump file. In Eclipse MAT, two types of object sizes are reported:.
The piechart shows the biggest objects by retained size in the overview tab as shown here:. For our application, this information in the overview means if we could dispose of a particular instance of java. Thread we will save 1. While that might look promising, java.
Thread is unlikely to be the real problem here. To get a better insight into what objects currently exist, we will use the Histogram view:. With this view, we can see the number of live objects: for example, BrandedProduct objects, and Price Objects are alive in the system. We can also see the amount of memory each object is using. There are two calculations, Shallow Heap and Retained Heap.
A shallow heap is the amount of memory consumed by one object. All the above said tools are part of JDK. In the next article, we will see the tools to analyze the heap dumps to detect the memory issues. Until then, stay tuned!!!
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