Java虚拟机类加载器及双亲委派机制

public Launcher() {    Launcher.ExtClassLoader var1;    try {        //初始化扩展类加载器，构造函数没有入参，无法获取启动类加载器        var1 = Launcher.ExtClassLoader.getExtClassLoader();    } catch (IOException var10) {        throw new InternalError("Could not create extension class loader", var10);    }
    try {        //初始化应用类加载器，入参为扩展类加载器        this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);    } catch (IOException var9) {        throw new InternalError("Could not create application class loader", var9);    }
    // 设置上下文类加载器    Thread.currentThread().setContextClassLoader(this.loader);       //...}

protected Class<?> loadClass(String name, boolean resolve)throws ClassNotFoundException{    //进行类加载操作时首先要加锁，避免并发加载    synchronized (getClassLoadingLock(name)) {        //首先判断指定类是否已经被加载过        Class<?> c = findLoadedClass(name);        if (c == null) {            long t0 = System.nanoTime();            try {                if (parent != null) {                    //如果当前类没有被加载且父类加载器不为null，则请求父类加载器进行加载操作                    c = parent.loadClass(name, false);                } else {                   //如果当前类没有被加载且父类加载器为null，则请求根类加载器进行加载操作                    c = findBootstrapClassOrNull(name);                }            } catch (ClassNotFoundException e) {            }
            if (c == null) {                long t1 = System.nanoTime();               //如果父类加载器加载失败，则由当前类加载器进行加载，                c = findClass(name);                //进行一些统计操作               // ...            }        }        //初始化该类        if (resolve) {            resolveClass(c);        }        return c;    }}

//建立数据库连接底层方法private static Connection getConnection(        String url, java.util.Properties info, Class<?> caller) throws SQLException {    //获取调用者的类加载器    ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;    synchronized(DriverManager.class) {        //由启动类加载器加载的类，该值为null，使用上下文类加载器        if (callerCL == null) {            callerCL = Thread.currentThread().getContextClassLoader();        }    }
    //...
    for(DriverInfo aDriver : registeredDrivers) {        //使用上下文类加载器去加载驱动        if(isDriverAllowed(aDriver.driver, callerCL)) {            try {                //加载成功，则进行连接                Connection con = aDriver.driver.connect(url, info);                //...            } catch (SQLException ex) {                if (reason == null) {                    reason = ex;                }            }        }         //...    }}

public class ClassLoaderTest extends ClassLoader {
  private String classPath;
  public ClassLoaderTest(String classPath) {    this.classPath = classPath;  }
  /**   * 编写findClass方法的逻辑   *   * @param name   * @return   * @throws ClassNotFoundException   */  @Override  protected Class<?> findClass(String name) throws ClassNotFoundException {    // 获取类的class文件字节数组    byte[] classData = getClassData(name);    if (classData == null) {      throw new ClassNotFoundException();    } else {      // 生成class对象      return defineClass(name, classData, 0, classData.length);    }  }
  /**   * 编写获取class文件并转换为字节码流的逻辑   *   * @param className   * @return   */  private byte[] getClassData(String className) {    // 读取类文件的字节    String path = classNameToPath(className);    try {      InputStream is = new FileInputStream(path);      ByteArrayOutputStream stream = new ByteArrayOutputStream();      byte[] buffer = new byte[2048];      int num = 0;      // 读取类文件的字节码      while ((num = is.read(buffer)) != -1) {        stream.write(buffer, 0, num);      }      return stream.toByteArray();    } catch (IOException e) {      e.printStackTrace();    }    return null;  }
  /**   * 类文件的完全路径   *   * @param className   * @return   */  private String classNameToPath(String className) {    return classPath + File.separatorChar        + className.replace('.', File.separatorChar) + ".class";  }
  public static void main(String[] args) {    String classPath = "/Users/zzs/my/article/projects/java-stream/src/main/java/";    ClassLoaderTest loader = new ClassLoaderTest(classPath);
    try {      //加载指定的class文件      Class<?> object1 = loader.loadClass("com.secbro2.classload.SubClass");      System.out.println(object1.newInstance().toString());    } catch (Exception e) {      e.printStackTrace();    }  }}