博客
关于我
JVM类加载器详解
阅读量:765 次
发布时间:2019-03-23

本文共 2985 字,大约阅读时间需要 9 分钟。

Java Class Loading Mechanism Explained

The Java class loading mechanism is a cornerstone of the Java Virtual Machine (JVM) that ensures applications run correctly by dynamically loading classes into memory. This process is managed by the ClassLoader class, which acts as a bridge between the external class files and the JVM's runtime environment.

The Class Loader Phases

The class loader operates through three main phases, each ensuring that classes are loaded, linked, and initialized correctly.

  • Loading Phase (Verification and Preparing)

    • Verification: The class loader reads the byte code from the file and checks it against the JVM specification to ensure it conforms to the expected structure. This prevents the loading of invalid or malformed class files.
    • Preparation: The class loader initializes the static variables and assigns default values to instance variables. This is done by the Class inappropriate class.
  • Linking Phase (Validation and Resolution)

    • Validation: The class loader cross-verifies the dynamic variables and ensures static fields are correctly resolved.
    • Resolution: The class loader converts symbolic references in the class file into direct references within the JVM. This is necessary for the JVM to efficiently access the classes and their members.
  • Initialization Phase

    • The class loader initializes the class by invoking its constructor. This process is managed by the JVM's clint method, which generates the initialization code dynamically based on the class structure.
  • Class Loader Types

    In the JVM, class loading is delegated to different class loaders. These include:

  • Bootstrap Class Loader (BootstrapClassLoader)

    • Written in C and responsible for loading core Java libraries (e.g., java.lang, javax, sun packages).
    • Operates in a isolated sandbox environment.
  • Extension Class Loader (ExtClassLoader)

    • Loads classes from the JAVA_HOME/jre/lib/ext directory. It allows third-party vendors to add their own classes by placing JAR files in this directory.
  • Application Class Loader (AppClassLoader)

    • Responsible for loading application classes from the classpath. It delegates loading requests to the Extension Class Loader unless a specific class is already loaded.
  • Double-Checked Locking Mechanism

    The class loader ensures that each class is loaded only once through the double-checked locking mechanism. This involves:

  • Checking if the class is already loaded.
  • If not, initiating the loading process.
  • Once the class is loaded, subsequent checks return the loaded class without re-loading.
  • This mechanism also supports the JVM's sandbox environment, preventing external classes from overriding core Java classes.

    Understanding the class loading mechanism is crucial for Java developers as it affects Everything from development to debugging, especially when dealing with classpath issues or performance optimizations.

    By mastery of the class loader architecture, you gain insight into how the JVM manages and enhances the dynamic nature of Java applications.

    转载地址:http://itozk.baihongyu.com/

    你可能感兴趣的文章
    MySQL中使用IN()查询到底走不走索引?
    查看>>
    Mysql中使用存储过程插入decimal和时间数据递增的模拟数据
    查看>>
    MySql中关于geometry类型的数据_空的时候如何插入处理_需用null_空字符串插入会报错_Cannot get geometry object from dat---MySql工作笔记003
    查看>>
    mysql中出现Incorrect DECIMAL value: '0' for column '' at row -1错误解决方案
    查看>>
    mysql中出现Unit mysql.service could not be found 的解决方法
    查看>>
    mysql中出现update-alternatives: 错误: 候选项路径 /etc/mysql/mysql.cnf 不存在 dpkg: 处理软件包 mysql-server-8.0的解决方法(全)
    查看>>
    Mysql中各类锁的机制图文详细解析(全)
    查看>>
    MySQL中地理位置数据扩展geometry的使用心得
    查看>>
    Mysql中存储引擎简介、修改、查询、选择
    查看>>
    Mysql中存储过程、存储函数、自定义函数、变量、流程控制语句、光标/游标、定义条件和处理程序的使用示例
    查看>>
    mysql中实现rownum,对结果进行排序
    查看>>
    mysql中对于数据库的基本操作
    查看>>
    Mysql中常用函数的使用示例
    查看>>
    MySql中怎样使用case-when实现判断查询结果返回
    查看>>
    Mysql中怎样使用update更新某列的数据减去指定值
    查看>>
    Mysql中怎样设置指定ip远程访问连接
    查看>>
    mysql中数据表的基本操作很难嘛,由这个实验来带你从头走一遍
    查看>>
    Mysql中文乱码问题完美解决方案
    查看>>
    mysql中的 +号 和 CONCAT(str1,str2,...)
    查看>>
    Mysql中的 IFNULL 函数的详解
    查看>>