Shulou(Shulou.com)06/01 Report--

Most people do not understand the knowledge points of this article "what is the role of string constant pool and buffer pool in JAVA", so the editor summarizes the following content, detailed content, clear steps, and has a certain reference value. I hope you can get something after reading this article. Let's take a look at this article "what is the role of string constant pool and buffer pool in JAVA".

1 constant pool

Constant pools are divided into two types, one is the static constant pool in the .class file, and the other is the runtime constant pool formed when the static constant pool in the .class file is loaded into JVM.

1.1 static constant pool

The constant pool in the .class file can be thought of as an array, where some constants are stored, and when you need to use this constant in a bytecode instruction, it is accessed through the index of the array.

Look at the following code:

String m = "hellohellohellohellohello"; String n = "hellohellohellohellohello"

It will look like this in bytecode:

/ / constant pool: # 1 hellohellohellohellohello # 2...-- String m = # 1; String n = # 1

Of course, this is only a simplified version, which is actually more complex (the actual version can see the answer posted in the Resources section at the end of the article. For now, you can only consider the simplified version).

Note that the string constant stored in this is just a simple UTF8-encoded byte sequence, not a Java string object, just like the string you store in an txt text. When we open a .class file in UTF8 format, we can see that hellohellohellohellohello can be parsed:

1.2 run-time pool

Once you understand the static constant pool, the runtime constant pool is easy to figure out. Simply put, a runtime constant pool is the runtime representation of a static constant pool in a .class file in JVM, and each static constant pool of a .class file generates a corresponding runtime pool. When JVM interprets the instruction String m = # 1, it can look up the definition of # 1 in the class's runtime pool.

2 string pool

String pool is a cache pool set by Java to reuse String objects. Before Java1.7 is set on the method area, String objects are saved; after Java1.7 is set on the heap, references to String objects are saved, and the String objects themselves exist in other locations on the heap. The following is based on the post-Java1.7 situation.

Continue with the above example. When JVM interprets String m = # 1, it has got the corresponding UTF8 sequence from the runtime pool. Next, it looks for the String object corresponding to the UTF8 sequence in the string pool and assigns a reference to m. You may wonder when this String was created. According to this article in R, if the corresponding object already exists in the string pool when the class in which this statement is loaded, then nothing will be done. If it does not exist, a corresponding String object will be created and its reference will be placed in the pool.

In addition to string pooling, Wrapper types such as Integer and Long also have their own cache pools. For example, Integer caches Integer objects from-128 to 127. When literal assignment or Integer.valueOf () is used, objects in the pool will be returned if there are corresponding objects in the pool, and new objects will be created on the heap only if they are not in the pool.

However, unlike string pools, these Wrapper pools do not grow as much as string pools, that is, the number of objects in the pool is fixed, and there are only-128 Integer 127 in the string pool.

The buffer pools for the basic types are as follows:

Boolean values true and false all byte values short values between-128 and 127int values between-128and 127char in the range\ u0000 to\ u007F

Among all the numeric buffer pools of jdk 1.8, Integer's buffer pool IntegerCache is very special. The lower bound of this buffer pool is-128and the upper bound is 127by default, but this upper bound is adjustable. When starting jvm, use-XX:AutoBoxCacheMax= to specify the size of the buffer pool. This option sets a system property called java.lang.IntegerCache.high when JVM initializes. Then when IntegerCache initializes, it reads the system properties to determine the upper bound.

The above is the content of this article on "what is the function of string constant pool and buffer pool in JAVA". I believe we all have a certain understanding. I hope the content shared by the editor will be helpful to you. If you want to know more about the relevant knowledge, please follow the industry information channel.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.