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-- 未来的魔法师
一个类在什么时候开始被加载，《Java虚拟机规范》中并没有进行强制约束，交给了虚拟机自己去自由实现。HotSpot虚拟机是按需加载，在需要用到该类的时候加载这个类。工程首先加载rt目录下的java基础类库，然后加载主程序类，然后用到什么类加载什么类。
创建Demo01为主类、Person为被调用类，执行main()方法，实例化Person类，并调用person类中的working()方法。具体实现如下代码：
public class Demo01 {
//常量
public static final int a =123;
//类变量
public static int b = 321;
//实例变量
public int abc;
// -XX:+TraceClassLoading 监控类的加载
public static void main(String[] args){
Person person = new Person();
person.working();
}
}
@Data
public class Person {
private int id;
//测试
public void working(){
}
}
结果日志打印内容如下所示：
- 首先加载jre rt文件下的rt依赖包
- 加载主程序类
- 加载被调用的person类
[Opened C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
//java核心类
[Loaded java.lang.Object from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.io.Serializable from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.lang.Comparable from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.lang.CharSequence from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.lang.String from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.lang.reflect.AnnotatedElement from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
//省略
..............
.............
.............
//加载编译后的程序class类
//加载主程序类
[Loaded Demo01 from file:/C:/code/java/java-tutorial/tutorials-in-java/interview-question/target/classes/]
[Loaded sun.launcher.LauncherHelper$FXHelper from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.net.URI from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.lang.Class$MethodArray from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.lang.Void from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded java.net.URI$Parser from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
[Loaded sun.net.spi.DefaultProxySelector$NonProxyInfo from C:\Program Files\Java\jdk1.8.0_241\jre\lib\rt.jar]
//加载被调用的person类
[Loaded Person from file:/C:/code/java/java-tutorial/tutorials-in-java/interview-question/target/classes/]
//补充：maven项目中导入的依赖如redis并未使用，不会被加载
