java类加载器
类加载器是一个用来加载类文件的类。Java源代码通过javac编译器编译成类文件。然后JVM来执行类文件中的字节码来执行程序。类加载器负责加载文件系统、网络或其他来源的类文件。
有三种默认使用的类加载器：Bootstrap类加载器、Extension类加载器和System类加载器（或者叫作Application类加载器）。每种类加载器所负责加载的类也各不相同。
Bootstrap ClassLoader : 将存放于<JAVA_HOME>\lib目录中的，或者被-Xbootclasspath参数所指定的路径中的，并且是虚拟机识别的（仅按照文件名识别，如 rt.jar 名字不符合的类库即使放在lib目录中也不会被加载）类库加载到虚拟机内存中。启动类加载器无法被Java程序直接引用
Extension ClassLoader : 将<JAVA_HOME>\lib\ext目录下的，或者被java.ext.dirs系统变量所指定的路径中的所有类库加载。开发者可以直接使用扩展类加载器。
Application ClassLoader : 负责加载用户类路径(ClassPath)上所指定的类库,开发者可直接使用。
类加载器的代理模式
类加载器在尝试自己去查找某个类的字节代码并定义它时，会先代理给其父类加载器，由父类加载器先去尝试加载这个类，依次类推。
在介绍代理模式背后的动机之前，首先需要说明一下 Java 虚拟机是如何判定两个 Java 类是相同的。Java 虚拟机不仅要看类的全名是否相同，还要看加载此类的类加载器是否一样。只有两者都相同的情况，才认为两个类是相同的。即便是同样的字节代码，被不同的类加载器加载之后所得到的类，也是不同的。比如一个 Java 类 com.example.Sample，编译之后生成了字节代码文件 Sample.class。两个不同的类加载器 ClassLoaderA和 ClassLoaderB分别读取了这个 Sample.class文件，并定义出两个 java.lang.Class类的实例来表示这个类。这两个实例是不相同的。对于 Java 虚拟机来说，它们是不同的类。试图对这两个类的对象进行相互赋值，会抛出运行时异常 ClassCastException。
一句话总结就是：查找某个类的时候从下至上，加载某个类的时候从上至下。
自定义类加载器
因为我们需要加载指定路径下的jar文件，所以我们需要自定义类加载器来扫描指定路径下的jar包，代码如下：package com.bytebeats.switcher.core;
import com.bytebeats.switcher.util.IoUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.io.File;
import java.io.FileFilter;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
/**
*
* @author Ricky Fung
* @create 2016-11-12 14:27
*/
public class PluginClassLoader {
private final Logger logger = LoggerFactory.getLogger(PluginClassLoader.class);
private URLClassLoader classLoader;
public PluginClassLoader(String jarfileDir){
this(new File(jarfileDir), null);
}
public PluginClassLoader(File jarfileDir){
this(jarfileDir, null);
}
public PluginClassLoader(File jarfileDir, ClassLoader parent) {
this.classLoader = createClassLoader(jarfileDir, parent);
}
public void addToClassLoader(final String baseDir, final FileFilter filter,
boolean quiet) {
File base = new File(baseDir);
if (base != null && base.exists() && base.isDirectory()) {
File[] files = base.listFiles(filter);
if (files == null || files.length == 0) {
if (!quiet) {
logger.error("No files added to classloader from lib: "
+ baseDir + " (resolved as: "
+ base.getAbsolutePath() + ").");
}
} else {
this.classLoader = replaceClassLoader(classLoader, base, filter);
}
} else {
if (!quiet) {
logger.error("Can't find (or read) directory to add to classloader: "
+ baseDir
+ " (resolved as: "
+ base.getAbsolutePath()
+ ").");
}
}
}
private URLClassLoader replaceClassLoader(final URLClassLoader oldLoader,
final File base, final FileFilter filter) {
if (null != base && base.canRead() && base.isDirectory()) {
File[] files = base.listFiles(filter);
if (null == files || 0 == files.length){
logger.error("replaceClassLoader base dir:{} is empty", base.getAbsolutePath());
return oldLoader;
}
logger.error("replaceClassLoader base dir: {} ,size: {}", base.getAbsolutePath(), files.length);
URL[] oldElements = oldLoader.getURLs();
URL[] elements = new URL[oldElements.length + files.length];
System.arraycopy(oldElements, 0, elements, 0, oldElements.length);
for (int j = 0; j < files.length; j++) {
try {
URL element = files[j].toURI().normalize().toURL();
elements[oldElements.length + j] = element;
logger.info("Adding '{}' to classloader", element.toString());
} catch (MalformedURLException e) {
logger.error("load jar file error", e);
}
}
ClassLoader oldParent = oldLoader.getParent();
IoUtils.closeQuietly(oldLoader); // best effort
return URLClassLoader.newInstance(elements, oldParent);
}
return oldLoader;
}
private URLClassLoader createClassLoader(final File libDir,
ClassLoader parent) {
if (null == parent) {
parent = Thread.currentThread().getContextClassLoader();
}
return replaceClassLoader(
URLClassLoader.newInstance(new URL[0], parent), libDir, null);
}
public Class<?> loadClass(String className) throws ClassNotFoundException{
return classLoader.loadClass(className);
}
} 为了方便使用， 提供了PluginManagerpackage com.bytebeats.switcher.core;
import com.bytebeats.switcher.util.StringUtils;
import java.io.File;
/**
* ${DESCRIPTION}
*
* @author Ricky Fung
* @create 2016-11-12 14:35
*/
public class PluginManager {
private volatile static PluginManager mgr;
private PluginClassLoader pluginClassLoader;
private volatile boolean init;
private PluginManager(){
}
public static PluginManager getMgr(){
if(mgr==null){
synchronized (PluginManager.class){
if(mgr==null){
mgr = new PluginManager();
}
}
}
return mgr;
}
public <T> T getPlugin(String className, Class<T> required){
Class<?> cls = null;
try {
cls = pluginClassLoader.loadClass(className);
} catch (ClassNotFoundException e) {
throw new IllegalArgumentException("can not find class:"+className, e);
}
if(required.isAssignableFrom(cls)){
try {
return (T) cls.newInstance();
} catch (Exception e) {
throw new IllegalArgumentException("can not newInstance class:"+className, e);
}
}
throw new IllegalArgumentException("class:"+className+" not sub class of "+required);
}
public void addExternalJar(String basePath){
if (StringUtils.isEmpty(basePath)) {
throw new IllegalArgumentException("basePath can not be empty!");
}
File dir = new File(basePath);
if(!dir.exists()){
throw new IllegalArgumentException("basePath not exists:"+basePath);
}
if(!dir.isDirectory()){
throw new IllegalArgumentException("basePath must be a directory:"+basePath);
}
if(!init){
init= true;
pluginClassLoader = doInit(basePath);
}else{
pluginClassLoader.addToClassLoader(basePath, null, true);
}
}
private synchronized PluginClassLoader doInit(String basePath){
PluginClassLoader pluginClassLoader = new PluginClassLoader(basePath);
return pluginClassLoader;
}
}插件式开发示例
example-api存放核心api接口，example-plugin是插件工程 提供example-api api接口实现，example-host是主程序 它在启动的时候会去加载plugin。
example-api中的api接口如下：package com.bytebeats.switcher.example.api;
/**
* ${DESCRIPTION}
*
* @author Ricky Fung
* @create 2016-11-12 15:30
*/
public interface IHelloService {
String hello(String msg);
}
package com.bytebeats.switcher.example.api;
import com.bytebeats.switcher.example.domain.User;
import java.util.List;
/**
* ${DESCRIPTION}
*
* @author Ricky Fung
* @create 2016-11-12 16:09
*/
public interface IUserService {
List<User> getUsers();
int update(User user);
}
clientpackage com.bytebeats.example.plugin;
import com.bytebeats.switcher.example.api.IHelloService;
/**
* ${DESCRIPTION}
*
* @author Ricky Fung
* @create 2016-11-12 16:13
*/
public class HelloServiceImpl implements IHelloService {
@Override
public String hello(String msg) {
System.out.println("hello [" + msg + "]");
return "hello [" + msg + "]";
}
} package com.bytebeats.example.plugin;
import com.bytebeats.switcher.example.api.IUserService;
import com.bytebeats.switcher.example.domain.User;
import java.util.ArrayList;
import java.util.List;
/**
* ${DESCRIPTION}
*
* @author Ricky Fung
* @create 2016-11-12 16:14
*/
public class UserServiceImpl implements IUserService {
@Override
public List<User> getUsers() {
List<User> list = new ArrayList<>();
list.add(new User("ricky", "12345"));
list.add(new User("kobe", "aaa"));
list.add(new User("jordan", "root"));
return list;
}
@Override
public int update(User user) {
System.out.println("update user = [" + user + "]");
user.setPassword("hello");
return 1;
}
} example-host中的启动类如下：package com.bytebeats.example.host;
import com.bytebeats.switcher.core.PluginManager;
import com.bytebeats.switcher.example.api.IHelloService;
import com.bytebeats.switcher.example.api.IUserService;
import com.bytebeats.switcher.example.domain.User;
import java.util.List;
/**
* Hello world!
*
*/
public class App {
public static void main( String[] args ) {
PluginManager pluginManager = PluginManager.getMgr();
pluginManager.addExternalJar("D:\\osgi");
IHelloService helloService = pluginManager.getPlugin("com.bytebeats.example.plugin.HelloServiceImpl", IHelloService.class);
helloService.hello("ricky");
IUserService userService = pluginManager.getPlugin("com.bytebeats.example.plugin.UserServiceImpl", IUserService.class);
List<User> list = userService.getUsers();
System.out.println("list = [" + list + "]");
userService.update(new User("test", "test"));
}
} 注意：运行之前需要将example-plugin工程打成jar包，然后拷贝到任意路径下即可，本文是D:\osgi目录下。GitHub： https://github.com/TiFG/cherry
