JVM内存模型（一）—— 类加载子系统

public static void main(String[] args) {
  URL[] urLs = Launcher.getBootstrapClassPath().getURLs();
  Stream.of(urLs).forEach(e-> System.out.println(e.toExternalForm()));
}

---------输出BootStrapClassLoad加载器加载的位置
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/lib/resources.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/lib/rt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/lib/sunrsasign.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/lib/jsse.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/lib/jce.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/lib/charsets.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/lib/jfr.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/classes

public static void main(String[] args) {
  String property = System.getProperty("java.ext.dirs");
  Stream.of(property.split(":")).forEach(System.out::println);
}

---------输出ExtClassLoad加载器加载的位置
/Users/babyshi/Library/Java/Extensions
/Library/Java/JavaVirtualMachines/jdk1.8.0_281.jdk/Contents/Home/jre/lib/ext
/Library/Java/Extensions
/Network/Library/Java/Extensions
/System/Library/Java/Extensions
/usr/lib/java

//方式一:获取当前类的ClassLoader
clazz.getClassLoader()
//方式二:获取当前线程上下文的ClassLoader
Thread.currentThread().getContextClassLoader()
//方式三:获取系统的ClassLoader
ClassLoader.getSystemClassLoader()
//方式四:获取调用者的ClassLoader
DriverManager.getCallerClassLoader()

public abstract class ClassLoader {

  // ....

  protected Class> loadClass(String name, boolean resolve) throws ClassNotFoundException {

    synchronized (getClassLoadingLock(name)) {
      // 1、检查该class对象是否已经被加载
      // First, check if the class has already been loaded
      Class> c = findLoadedClass(name);
      // 没有加载过的话进行加载Class
      if (c == null) {
        long t0 = System.nanoTime();
        try {
          // 存在父类加载器则调用父类的类加载器进行加载（此处是递归调用）
          if (parent != null) {
            c = parent.loadClass(name, false);
          } else {
            // 如果父加载器为空，查找Bootstrap加载器是不是加载过了
            c = findBootstrapClassOrNull(name);
          }
        } catch (ClassNotFoundException e) {
          // ClassNotFoundException thrown if class not found
          // from the non-null parent class loader
        }

        if (c == null) {
          // If still not found, then invoke findClass in order
          // to find the class.
          long t1 = System.nanoTime();
          // 如果父加载器没加载成功，调用自己的findClass去加载
          c = findClass(name);

          // this is the defining class loader; record the stats
          PerfCounter.getParentDelegationTime().addTime(t1 - t0);
          PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
          PerfCounter.getFindClasses().increment();
        }
      }
      if (resolve) {
        resolveClass(c);
      }
      return c;
    }
  }

  protected final Class> findLoadedClass(String name) {
    if (!checkName(name))
      return null;
    // 通过类名称查询 调用的native方法
    return findLoadedClass0(name);
  }

  protected Class> findClass(String name) throws ClassNotFoundException {
    throw new ClassNotFoundException(name);
    // 空实现，子类实现，主要是通过传入的类名name，到在特定目录下去寻找类文件，把.class文件读入内存
  }
}

package java.lang;

public class String {
    public static void main(String[] args) {
        System.out.println("hello Word");
    }
}

---------------报错内容如下：
错误: 在类 java.lang.String 中找不到 main 方法, 请将 main 方法定义为:
    public static void main(String[] args)
否则 JavaFX 应用程序类必须扩展javafx.application.Application