[Jinjiuyin 10 must-ask Java interview questions] During the six-year job interview, I was asked why the JVM replaced the permanent generation with metaspace?

"Why did the JVM replace the permanent generation with metaspace?"

This is a question that a classmate who has worked for 6 years encountered on the first side of byte. Unfortunately, he did not answer it.

Hi, my name is Mic, a Java programmer who has been working for 14 years.

How do we answer this question? What is the interviewer looking for?

Interview analysis

We all know that in Java8 and later versions, the structure of the JVM runtime data area is slowly being adjusted and optimized.

But in fact, these changes have no impact on business development partners.

So I can say that 99% of people can't answer this question.

However, the interview of the big Internet companies is to screen the 1% of outstanding talents, so through this question,

• It can not only test the job seeker's understanding of the principle of JVM
• It can also examine the solidity of the basic skills of job seekers
• It can also realize the screening of senior talents

In Java7, the JVM runtime data area is like this.

In the Hotspot virtual machine, the method area is implemented in the permanent generation, which mainly stores the runtime constant pool, Klass class meta information, etc.

The permanent generation belongs to a piece of storage space in the JVM runtime memory. We can set the size of the permanent generation through -XX:PermSize.

When there is not enough memory, garbage collection is triggered.

In JDK1.8, the JVM runtime data area is like this.

In the Hotspot virtual machine, the permanent generation is cancelled, and the data storage of the method area is realized by the meta space.

Metaspace does not belong to JVM memory, but directly uses local memory, so there is no need to consider GC issues.

By default, the metaspace can use unlimited local memory, but we can also use JVM parameters to limit the memory usage.

Why use metaspace to replace permanent generation, there must be a reason behind it, but if job seekers can answer it.

It is necessary to have a certain understanding of the underlying principles of the JVM.

Let's take a look at the expert's answer.

Expert:

There are three reasons:

• In version 1.7, the permanent generation memory has an upper limit. Although we can set it through parameters, it is difficult to determine the total number and size of classes loaded by the JVM.

  So it is easy to have OOM problems.

  However, the meta space is stored in the local memory, and the upper memory limit is relatively large, which can avoid this problem.

• The objects of the permanent generation are garbage collected through FullGC, that is, garbage collection is implemented at the same time as the old generation.

  After replacing with metaspace, Full GC is simplified. Class data can be released concurrently without pausing, while also improving GC performance

• Oracle wants to merge the code of Hotspot and JRockit, and JRockit has no permanent generation.

The above is my understanding of the problem.

Summarize

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