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This article will explain in detail the example analysis of the Java memory data area. The editor thinks it is very practical, so I share it for you as a reference. I hope you can get something after reading this article.

Mind map

There are only five areas in total (direct memory is not part of the JVM runtime data area). OOM (OutOfMemoryError) can appear everywhere except program counters, such as program counters and two stacks (Java virtual machine stack & local method stack), which are one for each thread, so it must be thread isolated. The other two zones are shared by threads, that is, if multiple threads want to access the data of these two zones at the same time, there will be thread safety problems. Next, we will introduce these areas in detail.

Program counter

The line number indicator of the bytecode executed by the current thread, and the bytecode interpreter works by changing the value of this counter to determine the location of the next bytecode instruction to be executed.

The difference when executing the Java method and the native method:

When executing the Java method: record the bytecode instruction address that the virtual machine is executing

When executing the native method: no definition

Is the only one of the five regions that does not have OOM.

Java virtual machine stack

The memory model executed by the Java method, and the process of each method execution is the process that the corresponding stack frame enters the stack to the unstack in the virtual machine stack.

Serving the Java method

Exceptions that may be thrown:

OutOfMemoryError (when the virtual machine stack can be expanded dynamically, enough memory cannot be applied for)

StackOverflowError (stack depth of thread requests > depth allowed by virtual machines)

Virtual machine parameter setting:-Xss.

Local method stack

Serving the native method

Possible exception thrown: same as the Java virtual machine stack.

Java reactor

Sole purpose: to store object instances

Main areas managed by garbage collector

Can be in physically discontinuous memory space

Exceptions that may be thrown:

OutOfMemoryError (there is no memory in the heap to allocate to the newly created instance, and the heap can no longer be expanded).

Virtual machine parameter settings:

Maximum:-Xmx

Minimum:-Xms

Setting the two parameters to the same value avoids automatic heap expansion.

Method area

Store data such as class information, constants, static variables, compiled code, etc. that have been loaded by the virtual machine.

Class information: that is, the Class class, such as class name, access modifier, constant pool, field description, method description, etc.

Garbage collection rarely occurs in this area.

However, we have to clean up. For applications that often generate a large number of Class dynamically, such as Spring, we need to pay special attention to the recovery of classes.

The runtime constant pool is also part of the method area.

In addition to the version, field, method, interface and other description information of the class in the Class file, there is also a constant pool, which is used to store various literals (that is, static final constants defined in the code) and symbolic references generated by the compiler. This part of the information is stored in the runtime pool.

Exceptions that may be thrown:

OutOfMemoryError (when the method zone cannot meet the memory allocation requirements).

Direct memory

The NIO class of JDK 1.4 can use the native function library to allocate out-of-heap memory directly, which is a channel-and-buffer-based Imax O approach, which stores a DirectByteBuffer object in the Java heap as a reference to out-of-heap memory, so that it can operate on out-of-heap memory. Because you can avoid copying data back and forth between the Java heap and the Native heap, there can be significant performance improvements in some scenarios.

Virtual machine parameter setting:-XX:MaxDirectMemorySize

The default is equal to the maximum value of the Java heap, which is the value specified by-Xmx.

The reason why it will be stored directly here is that it may also appear OutOfMemoryError.

Server administrators configure JVM parameters based on the machine's actual memory setting-Xmx and other information, but often ignore direct memory (the default is equal to the-Xmx setting), which may make the sum of each memory area greater than the physical memory limit, resulting in OOM during dynamic expansion.

This is the end of this article on "sample analysis of Java memory data areas". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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