JVM internal data structure: efficient management of Java objects

JVM internal data structures include object headers, class tables, and hash tables for efficient management of Java objects. The object header stores metadata, the class table provides class information, and the hash table enables fast object lookup, which together ensure high-performance operation of Java applications.

JVM internal data structure: efficient management of Java objects

Preface

JVM (Java Virtual Machine) is responsible for managing the execution environment of Java applications. In order to manage objects efficiently, the JVM uses a series of internal data structures. This article will take an in-depth look at these data structures and understand how they help the JVM store, access, and garbage collect objects efficiently.

Object header

Every Java object has a data area called "object header" in memory. The object header contains the following information:

• Mark Word: Indicates whether the object has been marked for garbage collection, as well as metadata such as lock status.
• Type pointer: Pointer to the class metadata information of the object.
• Instance data: The area where the actual data of the object is stored.

Class table

The class table is a data structure that stores information about loaded classes. It contains the following information:

• Class metadata: Class name, methods, fields and other information describing the class.
• Constant pool: Stores constant values ​​such as strings and values ​​related to the class.
• Method area: Stores the bytecode instructions of the class and other method-related information.

Hash table

The JVM uses a hash table to find objects quickly. A hash table is a data structure that allows fast access to data via a hash function. The JVM stores objects in different hash tables, depending on the object's type and access pattern.

Practical case

Consider the following Java code:

class Employee {
    private int id;
    private String name;
}

When an Employee object is created, the JVM will allocate a space in the memory and provide the object header and instance data assignment. The object header contains the object's tag bits, type pointer, and other metadata. Instance data stores the values ​​of the id and name attributes. The JVM will also look up the Employee class metadata in the class table and store it in the method area.

When it is necessary to access the properties of the Employee object, the JVM uses the type pointer in the object header to find the class metadata. It then uses the class metadata to determine the offsets of the id and name fields. Finally, the JVM reads the values ​​of these properties from the object's instance data area.

Conclusion

By using these internal data structures, the JVM can manage Java objects efficiently. Object headers, class tables, and hash tables enable the JVM to quickly store, access, and garbage collect objects, thereby ensuring high-performance operation of Java applications.

The above is the detailed content of JVM internal data structure: efficient management of Java objects. For more information, please follow other related articles on the PHP Chinese website!