自定义文件类加载器和网络类加载器

参考：https://blog.csdn.net/justloveyou_/article/details/72217806java

tomcat类加载详解：https://blog.csdn.net/justloveyou_/article/details/72231425web

1、文件类加载器spring

使用的类express

编译java类为class，移动到咱们想要放的目录，删除系统资源下的类，避免被appclassloader加载。apache

参考代码：数组

待测试类：Sample.java

package com.example;

public class Sample {

	private Sample instance;

	public void setSample(Object instance) {
		System.out.println(this.toString());
		System.out.println(this.getClass().getClassLoader());
		System.out.println(instance.toString());
		System.out.println(instance.getClass().getClassLoader());
		this.instance = (Sample) instance;
	}
}

文件加载器 FileSystemClassLoader.java

package classloader;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
//https://blog.csdn.net/justloveyou_/article/details/72217806
// 文件系统类加载器
public class FileSystemClassLoader extends ClassLoader {

	private String rootDir;

	public FileSystemClassLoader(String rootDir) {
		this.rootDir = rootDir;
	}

	// 获取类的字节码
	@Override
	protected Class<?> findClass(String name) throws ClassNotFoundException {
		byte[] classData = getClassData(name);  // 获取类的字节数组
		if (classData == null) {
			throw new ClassNotFoundException();
		} else {
			return defineClass(name, classData, 0, classData.length);
		}
	}

	private byte[] getClassData(String className) {
		// 读取类文件的字节
		String path = classNameToPath(className);
		try {
			InputStream ins = new FileInputStream(path);
			ByteArrayOutputStream baos = new ByteArrayOutputStream();
			int bufferSize = 4096;
			byte[] buffer = new byte[bufferSize];
			int bytesNumRead = 0;
			// 读取类文件的字节码
			while ((bytesNumRead = ins.read(buffer)) != -1) {
				baos.write(buffer, 0, bytesNumRead);
			}
			return baos.toByteArray();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return null;
	}

	private String classNameToPath(String className) {
		// 获得类文件的彻底路径
		return rootDir + File.separatorChar
				+ className.replace('.', File.separatorChar) + ".class";
	}
}

测试类：ClassIdentity.java

package classloader;

import java.lang.reflect.Method;

public class ClassIdentity {

	public static void main(String[] args) {
		new ClassIdentity().testClassIdentity();
	}

	public void testClassIdentity() {
		String classDataRootPath = "C:\\Users\\JackZhou\\Documents\\NetBeansProjects\\classloader\\build\\classes";
		classDataRootPath = "D:\\installdata\\indeaprojects\\demo\\src\\main\\java";
		FileSystemClassLoader fscl1 = new FileSystemClassLoader(classDataRootPath);
		FileSystemClassLoader fscl2 = new FileSystemClassLoader(classDataRootPath);
		String className = "com.example.Sample";
		try {
			Class<?> class1 = fscl1.loadClass(className);  // 加载Sample类
			Object obj1 = class1.newInstance();  // 建立对象
			Class<?> class2 = fscl2.loadClass(className);//改了fscl2为fscl1不报错
			Object obj2 = class2.newInstance();
			Method setSampleMethod = class1.getMethod("setSample", java.lang.Object.class);
			setSampleMethod.invoke(obj1, obj2);//不是一个classloder加载的不能互转，fscl2改成fscl1就不会报错
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

com.example.Sample@6b884d57
classloader.FileSystemClassLoader@85ede7b
com.example.Sample@38af3868
classloader.FileSystemClassLoader@63961c42
java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at classloader.ClassIdentity.testClassIdentity(ClassIdentity.java:23)
	at classloader.ClassIdentity.main(ClassIdentity.java:8)
Caused by: java.lang.ClassCastException: com.example.Sample cannot be cast to com.example.Sample
	at com.example.Sample.setSample(Sample.java:12)
	... 6 more

若是改成同一个类加载器,发现类实例对象不同，能够互转

com.example.Sample@65b54208
classloader.FileSystemClassLoader@85ede7b
com.example.Sample@1be6f5c3
classloader.FileSystemClassLoader@85ede7b

2、网络类加载器tomcat

使用的类springboot

我使用的容器是tomcat，框架是springboot2，maven3.6, jdk1.8 构建工程，提供服务网络

代码：app

个人资源路径（file:D:/installdata/indeaprojects/demo/target/classes/）

WebConfig.java (对外提供本地资源)

package com.config;

	import org.springframework.boot.web.servlet.MultipartConfigFactory;
	import org.springframework.context.annotation.Bean;
	import org.springframework.context.annotation.Configuration;
	import org.springframework.util.unit.DataSize;
	import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
	import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

	import javax.servlet.MultipartConfigElement;

	/**
	 * description: //拦截器
	 * https://blog.csdn.net/allen_lu_hh/article/details/83218028
	 * @author: heliming
	 * @date:2019/04/14 下午 6:04
	 */
	@Configuration
	public class WebConfig implements WebMvcConfigurer {
		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {

			registry.addResourceHandler("/classes/**").addResourceLocations("file:D:/installdata/indeaprojects/demo/target/classes/");

		}

		/**
		 * 设置单个文件上传大小限制为10M(拍照上传头像文件超限)
		 */
		@Bean
		public MultipartConfigElement getMultipartConfigElement() {
			MultipartConfigFactory factory = new MultipartConfigFactory();
			factory.setMaxFileSize(DataSize.ofBytes(10 * 1024 * 1024));
			return factory.createMultipartConfig();
		}
	}

客户端使用的接口

ICalculator.java

package classloader;

	public interface ICalculator extends Versioned {

		String calculate(String expression);
	}

Versioned.java

package classloader;

	public interface Versioned {

		String getVersion();
	}

服务端对接口实现（须要引用客户端的接口，jar包方式或者class文件方式）

CalculatorAdvanced.java

package com.example;

	import classloader.ICalculator;

	public class CalculatorAdvanced implements ICalculator {

		@Override
		public String calculate(String expression) {
			return "Result is " + expression;
		}

		@Override
		public String getVersion() {
			return "2.0";
		}
	}

CalculatorBasic.java

package com.example;

	import classloader.ICalculator;

	public class CalculatorBasic implements ICalculator {

		@Override
		public String calculate(String expression) {
			return expression;
		}

		@Override
		public String getVersion() {
			return "1.0";
		}
	}

HangxinApplication.java（服务端启动类）

package com;

	import org.springframework.boot.SpringApplication;
	import org.springframework.boot.autoconfigure.SpringBootApplication;

	@SpringBootApplication
	public class HangxinApplication {

		public static void main(String[] args) {
			SpringApplication.run(HangxinApplication.class, args);
		}

	}

application.yml

server:
	  port: 8888

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
	<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
			 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
		<modelVersion>4.0.0</modelVersion>
		<parent>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-parent</artifactId>
			<version>2.4.1</version>
			<relativePath/> <!-- lookup parent from repository -->
		</parent>
		<groupId>com.example</groupId>
		<artifactId>demo</artifactId>
		<version>0.0.1-SNAPSHOT</version>
		<name>demo</name>
		<description>Demo project for Spring Boot</description>

		<properties>
			<java.version>1.8</java.version>
		</properties>

		<dependencies>
			<dependency>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-starter-web</artifactId>
			</dependency>

			<dependency>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-starter-test</artifactId>
				<scope>test</scope>
			</dependency>
			<!--thymeleaf-->
			<dependency>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-starter-thymeleaf</artifactId>
			</dependency>
		</dependencies>

		<build>
			<plugins>
				<plugin>
					<groupId>org.springframework.boot</groupId>
					<artifactId>spring-boot-maven-plugin</artifactId>
				</plugin>
			</plugins>
		</build>

	</project>

客户端代码：

启动类CalculatorTest.java

package classloader;

	public class CalculatorTest {

		public static void main(String[] args) {

			String url = "http://10.0.21.168:8888/classes";
			NetworkClassLoader ncl = new NetworkClassLoader(url);
			String basicClassName = "com.example.CalculatorBasic";
			String advancedClassName = "com.example.CalculatorAdvanced";
			try {
				Class<?> clazz = ncl.loadClass(basicClassName);  // 加载一个版本的类
				ICalculator calculator = (ICalculator) clazz.newInstance();  // 建立对象
				System.out.println(calculator.getVersion());
				clazz = ncl.loadClass(advancedClassName);  // 加载另外一个版本的类
				calculator = (ICalculator) clazz.newInstance();
				System.out.println(calculator.getVersion());
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}

接口1 ICalculator.java

package classloader;


	public interface ICalculator extends Versioned {

		String calculate(String expression);
	}

接口2 Versioned.java

package classloader;

	public interface Versioned {

		String getVersion();
	}

网络类加载器 NetworkClassLoader.java

package classloader;

	import java.io.ByteArrayOutputStream;
	import java.io.InputStream;
	import java.net.URL;

	public class NetworkClassLoader extends ClassLoader {

		private String rootUrl;

		public NetworkClassLoader(String rootUrl) {
			// 指定URL
			this.rootUrl = rootUrl;
		}

		// 获取类的字节码
		@Override
		protected Class<?> findClass(String name) throws ClassNotFoundException {
			byte[] classData = getClassData(name);
			if (classData == null) {
				throw new ClassNotFoundException();
			} else {
				return defineClass(name, classData, 0, classData.length);
			}
		}

		private byte[] getClassData(String className) {
			// 从网络上读取的类的字节
			String path = classNameToPath(className);
			System.out.println(path);
			try {
				URL url = new URL(path);
				InputStream ins = url.openStream();
				ByteArrayOutputStream baos = new ByteArrayOutputStream();
				int bufferSize = 4096;
				byte[] buffer = new byte[bufferSize];
				int bytesNumRead = 0;
				// 读取类文件的字节
				while ((bytesNumRead = ins.read(buffer)) != -1) {
					baos.write(buffer, 0, bytesNumRead);
				}
				return baos.toByteArray();
			} catch (Exception e) {
				e.printStackTrace();
			}
			return null;
		}

		private String classNameToPath(String className) {
			// 获得类文件的URL
			return rootUrl + "/"
					+ className.replace('.', '/') + ".class";
		}
	}

测试代码：启动服务端的springboot，而后启动客户端，发现客户端能够加载服务端的代码。

http://10.0.21.168:8888/classes/com/example/CalculatorBasic.class
	1.0
	http://10.0.21.168:8888/classes/com/example/CalculatorAdvanced.class
	2.0