热替换实战

待我称王封你为后i 2022-09-06 14:20 109阅读 0赞

# 一 代码结构图 #

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

## 二 代码 ##

## 1 被动态替换的类 ##

package chapter04.java1;
    
    /**
    * replaced class
    */
    public class Demo1 {
        public void hot() {
            // System.out.println("OldDemo1"); // A:old class print
            // System.out.println("NewDemo1"); // B:new class print
        }
    }

## 2 自定义类加载器 ##

package chapter04.java1;
    
    import java.io.*;
    import java.nio.ByteBuffer;
    import java.nio.channels.Channels;
    import java.nio.channels.FileChannel;
    import java.nio.channels.WritableByteChannel;
    
    /**
    * 自定义类的加载器
    */
    public class MyClassLoader extends ClassLoader {
        private String rootDir;
    
        public MyClassLoader(String rootDir) {
            this.rootDir = rootDir;
        }
    
        protected Class<?> findClass(String className) throws ClassNotFoundException {
            Class clazz = this.findLoadedClass(className);
            FileChannel fileChannel = null;
            WritableByteChannel outChannel = null;
            if (null == clazz) {
                try {
                    String classFile = getClassFile(className);
                    FileInputStream fis = new FileInputStream(classFile);
                    fileChannel = fis.getChannel();
                    ByteArrayOutputStream baos = new ByteArrayOutputStream();
                    outChannel = Channels.newChannel(baos);
                    ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
                    while (true) {
                        int i = fileChannel.read(buffer);
                        if (i == 0 || i == -1) {
                            break;
                        }
                        buffer.flip();
                        outChannel.write(buffer);
                        buffer.clear();
                    }
    
                    byte[] bytes = baos.toByteArray();
                    clazz = defineClass(className, bytes, 0, bytes.length);
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    try {
                        if (fileChannel != null)
                            fileChannel.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    try {
                        if (outChannel != null)
                            outChannel.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
            return clazz;
        }
    
        /**
         * 类文件的完全路径
         */
        private String getClassFile(String className) {
            return rootDir + "/" + className.replace('.', '/') + ".class";
        }
    }

## 3 测试类 ##

package chapter04.java1;
    
    import java.lang.reflect.Method;
    
    public class LoopRun {
        public static void main(String args[]) {
            while (true) {
                try {
                    // 1. 创建自定义类加载器的实例
                    String rootDir = "E:\\jvmp2\\";
                    MyClassLoader loader = new MyClassLoader(rootDir);
                    // 2. 加载指定的类
                    Class clazz = loader.findClass("chapter04.java1.Demo1");
                    // 3. 创建运行时类的实例
                    Object demo = clazz.newInstance();
                    // 4. 获取运行时类中指定的方法
                    Method m = clazz.getMethod("hot");
                    // 5. 调用指定的方法
                    m.invoke(demo);
                    Thread.sleep(5000);
                } catch (Exception e) {
                    System.out.println("not find");
                    try {
                        Thread.sleep(5000);
                    } catch (InterruptedException ex) {
                        ex.printStackTrace();
                    }
                }
            }
        }
    }

# 三 测试 #

## 1 仅让 Demo1 的 B 处代码生效，编译 Demo1.java ##

> E:\\jvmp2\\chapter04\\java1>javac Demo1.java

## 2 运行 main，此时控制台打印如下  ##

> NewDemo1
> 
> NewDemo1
> 
> NewDemo1
> 
> NewDemo1

## 3 过上一段时间，仅让 Demo1 的 A 处代码生效，编译 Demo1.java，此时控制台打印如下 ##

> NewDemo1
> 
> NewDemo1
> 
> NewDemo1
> 
> NewDemo1
> 
> OldDemo1
> 
> OldDemo1
> 
> OldDemo1
> 
> OldDemo1

## 4 打印发生了变化，说明热替换生效。 ##

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70]: /images/20220829/7c29249991da4014a703d69ed79c26e1.png