


In-depth analysis of the understanding and role of string constant pool and buffer pool in JAVA
This article brings you relevant knowledge about java, which mainly introduces the understanding and role of string constant pool and buffer pool. String pool and constant pool are completely different. There are two things, but many places like to confuse them, which can easily cause misunderstandings among beginners. Here I would like to discuss them carefully, hoping to help everyone.
Recommended study: "java Learning Tutorial"
String pool can also be calledString constant pool , I think this name is the source of misunderstanding. Some people omit the three words string, leaving only constant pool.. . So in order to avoid misunderstanding, I suggest that when referring to the cache pool of string objects, just call it String pool
1 Constant pool
Constant The pool is divided into two types, one is the static constant pool in the .class file, and the other is the runtime constant pool# formed by loading the static constant pool in the .class file into the JVM. ##.
1.1 Static constant pool
The constant pool in the .class file can be regarded as an array. Some constants are stored in the array. When needed in the bytecode instructions When this constant is used, 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 the bytecode:
// 常量池: #1 hellohellohellohellohello #2 ... ... ---------------------------- String m = #1; String n = #1;Of course, this is just a simplified version, in fact More complicated (for the actual version, you can see the answer posted in the reference 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 An encoded byte sequence, not a Java string object. It is the same as the string you store in a txt text. We use UTF8 format to open a .class file and you can see
hellohellohellohellohello It can be parsed:
1.2 Runtime constant pool
After understanding the static constant pool, the runtime constant pool is easy to understand. . Simply put, the runtime constant pool is the runtime representation of the static constant pool in the .class file in the JVM. Each static constant pool in the .class file will generate a corresponding runtime constant pool. When the JVM interprets the instructionString m = #1, it can go to the runtime constant pool of this class to find the definition of #1.
String object. It was set in the method area before Java 1.7 and saved It is a String object; it is set on the heap after Java 1.7 and saves a reference to the
String object. The
String object itself exists elsewhere on the heap. The following takes the situation after Java 1.7 as the standard.
Integer and
Long also have their own cache pools. For example,
Integer will cache from -128~127 The
Integer object, when using literal assignment or
Integer.valueOf(), if the corresponding object exists in the pool, the object in the pool will be returned. Only when there is no such object in the pool Only then will new objects be created on the heap.
boolean values true and false all byte values short values between -128 and 127 int values between -128 and 127 char in the range \u0000 to \u007FAmong all the numerical buffer pools in jdk 1.8, the Integer buffer pool IntegerCache is very special. The lower bound of this buffer pool is - 128, upper The default limit is 127, but this upper limit is adjustable. When starting the jvm, specify the size of this buffer pool through -XX:AutoBoxCacheMax=. This option will set a file named java.lang when the JVM is initialized. .IntegerCache.high system property, and then when IntegerCache is initialized, it will read the system property to determine the upper bound. Recommended study: "
java tutorial"
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