07. 自定义类加载器

第一个自定义类加载器

1. 继承抽象类ClassLoader

   public class MyTest16 extends ClassLoader{
   }
   

2. 定义classLoaderName，fileExtension两个成员变量

   public class MyTest16 extends ClassLoader{
   
       // 类加载器名字
       private String classLoaderName;
   
       // 字节码文件扩展名
       private final String fileExtension = ".class";
   }
   

3. 定义两个构造方法

   public class MyTest16 extends ClassLoader{
   
       // 类加载器名字
       private String classLoaderName;
   
       // 字节码文件扩展名
       private final String fileExtension = ".class";
   
       public MyTest16(String classLoaderName) {
           // 使用getSystemClassLoader()返回的类加载器，即AppClassLoader作为该类加载器的父类加载器
           super();
           this.classLoaderName = classLoaderName;
       }
   
       public MyTest16(ClassLoader parent, String classLoaderName) {
           // 显式指定该类加载器的父类加载器
           super(parent);
           this.classLoaderName = classLoaderName;
       }
   }
   

4. 定义loadClassData()方法

   public class MyTest16 extends ClassLoader{
   
       // 类加载器名字
       private String classLoaderName;
   
       // 字节码文件扩展名
       private final String fileExtension = ".class";
   
       public MyTest16(String classLoaderName) {
           // 使用getSystemClassLoader()返回的类加载器，即AppClassLoader作为该类加载器的父类加载器
           super();
           this.classLoaderName = classLoaderName;
       }
   
       public MyTest16(ClassLoader parent, String classLoaderName) {
           // 显式指定该类加载器的父类加载器
           super(parent);
           this.classLoaderName = classLoaderName;
       }
   
       private byte[] loadClassData(String className) {
           InputStream is = null;
           byte[] data = null;
           ByteArrayOutputStream baos = null;
   
           try {
               is = new FileInputStream(new File(className + this.fileExtension));
               baos = new ByteArrayOutputStream();
               int ch = 0;
               while (-1 != (ch = is.read())){
                   baos.write(ch);
               }
               data = baos.toByteArray();
           }catch (Exception e) {
               e.printStackTrace();
           }finally {
               try {
                   is.close();
                   baos.close();
               }catch (Exception e) {
                   e.printStackTrace();
               }
           }
           return data;
       }
   
       @Override
       public String toString() {
           return "[" + this.classLoaderName + "]";
       }
   }
   

5. 重写findClass()方法

   public class MyTest16 extends ClassLoader{
   
       // 类加载器名字
       private String classLoaderName;
   
       // 字节码文件扩展名
       private final String fileExtension = ".class";
   
       public MyTest16(String classLoaderName) {
           // 使用getSystemClassLoader()返回的类加载器，即AppClassLoader作为该类加载器的父类加载器
           super();
           this.classLoaderName = classLoaderName;
       }
   
       public MyTest16(ClassLoader parent, String classLoaderName) {
           // 显式指定该类加载器的父类加载器
           super(parent);
           this.classLoaderName = classLoaderName;
       }
   
       @Override
       protected Class<?> findClass(String className) throws ClassNotFoundException {
           byte[] data = this.loadClassData(className);
           return this.defineClass(className, data, 0, data.length);
       }
   
       private byte[] loadClassData(String className) {
           InputStream is = null;
           byte[] data = null;
           ByteArrayOutputStream baos = null;
   
           try {
               is = new FileInputStream(new File(className + this.fileExtension));
               baos = new ByteArrayOutputStream();
               int ch = 0;
               while (-1 != (ch = is.read())){
                   baos.write(ch);
               }
               data = baos.toByteArray();
           }catch (Exception e) {
               e.printStackTrace();
           }finally {
               try {
                   is.close();
                   baos.close();
               }catch (Exception e) {
                   e.printStackTrace();
               }
           }
           return data;
       }
   
       @Override
       public String toString() {
           return "[" + this.classLoaderName + "]";
       }
   }
   

定义第二个类加载器

在第一个类加载器基础上添加path字段,从什么地方加载 绝对路径

在findClass里面添加两个输出,这两个信息输出 说明findClass()方法被调用

public class MyTest16_2 extends ClassLoader{

    // 类加载器名字
    private String classLoaderName;

    // 新增字段path 从什么地方加载 绝对路径
    private String path;

    // 新增set方法
    public void setPath(String path) {
        this.path = path;
    }

    // 字节码文件扩展名
    private final String fileExtension = ".class";

    public MyTest16_2(String classLoaderName) {
        // 使用getSystemClassLoader()返回的类加载器，即AppClassLoader作为该类加载器的父类加载器
        super();
        this.classLoaderName = classLoaderName;
    }

    public MyTest16_2(ClassLoader parent, String classLoaderName) {
        // 显式指定该类加载器的父类加载器
        super(parent);
        this.classLoaderName = classLoaderName;
    }

    @Override
    protected Class<?> findClass(String className) throws ClassNotFoundException {
        // 这两个信息输出 说明findClass()方法被调用
        System.out.println("findClass invoked: " + className);
        System.out.println("class loader name: " + this.classLoaderName);

        byte[] data = this.loadClassData(className);
        return this.defineClass(className, data, 0, data.length);
    }

    private byte[] loadClassData(String className) {
        InputStream is = null;
        byte[] data = null;
        ByteArrayOutputStream baos = null;

        // windows 系统 替换成\\
        className = className.replace(".", "/");

        try {
            is = new FileInputStream(new File( this.path + className + this.fileExtension));
            baos = new ByteArrayOutputStream();
            int ch = 0;
            while (-1 != (ch = is.read())){
                baos.write(ch);
            }
            data = baos.toByteArray();
        }catch (Exception e) {
            e.printStackTrace();
        }finally {
            try {
                is.close();
                baos.close();
            }catch (Exception e) {
                e.printStackTrace();
            }
        }
        return data;
    }

    @Override
    public String toString() {
        return "[" + this.classLoaderName + "]";
    }
}