前言
如果用过 JDK 提供的 SPI 机制的敌人，大略就会晓得它无奈按需加载。之前写过一篇文章聊聊基于 jdk 实现的 spi 如何与 spring 整合实现依赖注入。利用 spring 的依赖注入来实现 spi 按需加载，这种计划就是要借用 spring。明天咱们在聊聊另外一种实现形式，就是咱们本人手写一个
实现思路
整体思路和 jdk 实现 spi 差不多，如果对 jdk 实现的 spi 不理解，能够查看我之前写的文章 java 之 spi 机制简介。差异就是咱们在配置文件是以 key-value 的模式存在，形如
springMysql=com.github.lybgeek.dialect.mysql.SpringMysqlDialect实现逻辑
1、约定好要进行解析的目录，比方 META-INF/services/
private static final String SERVICE_DIRECTORY = "META-INF/services/";2、约定好要解析的文件名命名，比方
com.github.lybgeek.dialect.SpringSqlDialect3、约定好文件内容格局，比方
springMysql=com.github.lybgeek.dialect.mysql.SpringMysqlDialect4、获取约定好的目录，解析文件，并将相应内容放入缓存
/**
* Load files under SERVICE_DIRECTORY.
*/
private void loadDirectory(final Map<String, Class<?>> classes) {String fileName = SERVICE_DIRECTORY + clazz.getName();
try {ClassLoader classLoader = ExtensionLoader.class.getClassLoader();
Enumeration<URL> urls = classLoader != null ? classLoader.getResources(fileName)
: ClassLoader.getSystemResources(fileName);
if (urls != null) {while (urls.hasMoreElements()) {URL url = urls.nextElement();
loadResources(classes, url);
}
}
} catch (IOException t) {log.error("load extension class error {}", fileName, t);
}
}
private void loadResources(final Map<String, Class<?>> classes, final URL url) throws IOException {try (InputStream inputStream = url.openStream()) {Properties properties = new Properties();
properties.load(inputStream);
properties.forEach((k, v) -> {String name = (String) k;
String classPath = (String) v;
if (StringUtils.isNotBlank(name) && StringUtils.isNotBlank(classPath)) {
try {loadClass(classes, name, classPath);
} catch (ClassNotFoundException e) {throw new IllegalStateException("load extension resources error", e);
}
}
});
} catch (IOException e) {throw new IllegalStateException("load extension resources error", e);
}
}
private void loadClass(final Map<String, Class<?>> classes,
final String name, final String classPath) throws ClassNotFoundException {Class<?> subClass = Class.forName(classPath);
if (!clazz.isAssignableFrom(subClass)) {throw new IllegalStateException("load extension resources error," + subClass + "subtype is not of" + clazz);
}
Activate annotation = subClass.getAnnotation(Activate.class);
if (annotation == null) {throw new IllegalStateException("load extension resources error," + subClass + "with Activate annotation");
}
Class<?> oldClass = classes.get(name);
if (oldClass == null) {classes.put(name, subClass);
} else if (oldClass != subClass) {throw new IllegalStateException("load extension resources error,Duplicate class" + clazz.getName() + "name" + name + "on" + oldClass.getName() + "or" + subClass.getName());
}
}
5、依据 key，去缓存查找相应的类实例
public T getActivate(final String name) {if (StringUtils.isBlank(name)) {throw new NullPointerException("get Activate name is null");
}
Holder<Object> objectHolder = cachedInstances.get(name);
if (objectHolder == null) {cachedInstances.putIfAbsent(name, new Holder<>());
objectHolder = cachedInstances.get(name);
}
Object value = objectHolder.getValue();
if (value == null) {synchronized (cachedInstances) {value = objectHolder.getValue();
if (value == null) {value = createExtension(name);
objectHolder.setValue(value);
}
}
}
return (T) value;
}外围代码
@Slf4j
@SuppressWarnings("all")
public final class ExtensionLoader<T> {
private static final String SERVICE_DIRECTORY = "META-INF/services/";
private static final Map<Class<?>, ExtensionLoader<?>> LOADERS = new ConcurrentHashMap<>();
private final Class<T> clazz;
private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<>();
private final Map<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<>();
private final Map<Class<?>, Object> ActivateInstances = new ConcurrentHashMap<>();
private String cachedDefaultName;
/**
* Instantiates a new Extension loader.
*
* @param clazz the clazz.
*/
private ExtensionLoader(final Class<T> clazz) {
this.clazz = clazz;
if (clazz != ExtensionFactory.class) {ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getExtensionClasses();}
}
/**
* Gets extension loader.
*
* @param <T> the type parameter
* @param clazz the clazz
* @return the extension loader.
*/
public static <T> ExtensionLoader<T> getExtensionLoader(final Class<T> clazz) {if (clazz == null) {throw new NullPointerException("extension clazz is null");
}
if (!clazz.isInterface()) {throw new IllegalArgumentException("extension clazz (" + clazz + ") is not interface!");
}
if (!clazz.isAnnotationPresent(SPI.class)) {throw new IllegalArgumentException("extension clazz (" + clazz + ") without @" + SPI.class + "Annotation");
}
ExtensionLoader<T> extensionLoader = (ExtensionLoader<T>) LOADERS.get(clazz);
if (extensionLoader != null) {return extensionLoader;}
LOADERS.putIfAbsent(clazz, new ExtensionLoader<>(clazz));
return (ExtensionLoader<T>) LOADERS.get(clazz);
}
/**
* Gets default Activate.
*
* @return the default Activate.
*/
public T getDefaultActivate() {getExtensionClasses();
if (StringUtils.isBlank(cachedDefaultName)) {return null;}
return getActivate(cachedDefaultName);
}
/**
* Gets Activate.
*
* @param name the name
* @return the Activate.
*/
public T getActivate(final String name) {if (StringUtils.isBlank(name)) {throw new NullPointerException("get Activate name is null");
}
Holder<Object> objectHolder = cachedInstances.get(name);
if (objectHolder == null) {cachedInstances.putIfAbsent(name, new Holder<>());
objectHolder = cachedInstances.get(name);
}
Object value = objectHolder.getValue();
if (value == null) {synchronized (cachedInstances) {value = objectHolder.getValue();
if (value == null) {value = createExtension(name);
objectHolder.setValue(value);
}
}
}
return (T) value;
}
public Set<String> getSupportedExtensions() {Map<String, Class<?>> clazzes = getExtensionClasses();
return Collections.unmodifiableSet(new TreeSet<>(clazzes.keySet()));
}
@SuppressWarnings("unchecked")
private T createExtension(final String name) {Class<?> aClass = getExtensionClasses().get(name);
if (aClass == null) {throw new IllegalArgumentException("name is error");
}
Object o = ActivateInstances.get(aClass);
if (o == null) {
try {ActivateInstances.putIfAbsent(aClass, aClass.newInstance());
o = ActivateInstances.get(aClass);
} catch (InstantiationException | IllegalAccessException e) {
throw new IllegalStateException("Extension instance(name:" + name + ", class:"
+ aClass + ") could not be instantiated:" + e.getMessage(), e);
}
}
return (T) o;
}
/**
* Gets extension classes.
*
* @return the extension classes
*/
public Map<String, Class<?>> getExtensionClasses() {Map<String, Class<?>> classes = cachedClasses.getValue();
if (classes == null) {synchronized (cachedClasses) {classes = cachedClasses.getValue();
if (classes == null) {classes = loadExtensionClass();
cachedClasses.setValue(classes);
}
}
}
return classes;
}
private Map<String, Class<?>> loadExtensionClass() {SPI annotation = clazz.getAnnotation(SPI.class);
if (annotation != null) {String value = annotation.value();
if (StringUtils.isNotBlank(value)) {cachedDefaultName = value;}
}
Map<String, Class<?>> classes = new HashMap<>(16);
loadDirectory(classes);
return classes;
}
/**
* Load files under SERVICE_DIRECTORY.
*/
private void loadDirectory(final Map<String, Class<?>> classes) {String fileName = SERVICE_DIRECTORY + clazz.getName();
try {ClassLoader classLoader = ExtensionLoader.class.getClassLoader();
Enumeration<URL> urls = classLoader != null ? classLoader.getResources(fileName)
: ClassLoader.getSystemResources(fileName);
if (urls != null) {while (urls.hasMoreElements()) {URL url = urls.nextElement();
loadResources(classes, url);
}
}
} catch (IOException t) {log.error("load extension class error {}", fileName, t);
}
}
private void loadResources(final Map<String, Class<?>> classes, final URL url) throws IOException {try (InputStream inputStream = url.openStream()) {Properties properties = new Properties();
properties.load(inputStream);
properties.forEach((k, v) -> {String name = (String) k;
String classPath = (String) v;
if (StringUtils.isNotBlank(name) && StringUtils.isNotBlank(classPath)) {
try {loadClass(classes, name, classPath);
} catch (ClassNotFoundException e) {throw new IllegalStateException("load extension resources error", e);
}
}
});
} catch (IOException e) {throw new IllegalStateException("load extension resources error", e);
}
}
private void loadClass(final Map<String, Class<?>> classes,
final String name, final String classPath) throws ClassNotFoundException {Class<?> subClass = Class.forName(classPath);
if (!clazz.isAssignableFrom(subClass)) {throw new IllegalStateException("load extension resources error," + subClass + "subtype is not of" + clazz);
}
Activate annotation = subClass.getAnnotation(Activate.class);
if (annotation == null) {throw new IllegalStateException("load extension resources error," + subClass + "with Activate annotation");
}
Class<?> oldClass = classes.get(name);
if (oldClass == null) {classes.put(name, subClass);
} else if (oldClass != subClass) {throw new IllegalStateException("load extension resources error,Duplicate class" + clazz.getName() + "name" + name + "on" + oldClass.getName() + "or" + subClass.getName());
}
}
/**
* The type Holder.
*
* @param <T> the type parameter.
*/
public static class Holder<T> {
private volatile T value;
/**
* Gets value.
*
* @return the value
*/
public T getValue() {return value;}
/**
* Sets value.
*
* @param value the value
*/
public void setValue(final T value) {this.value = value;}
}
}
应用示例
1、定义服务接口
@SPI("mysql")
public interface SqlDialect {String dialect();
}
2、定义具体实现类
@Activate
public class MysqlDialect implements SqlDialect {
@Override
public String dialect() {return "mysql";}
}
@Activate
public class OracleDialect implements SqlDialect {
@Override
public String dialect() {return "oracle";}
}
3、src/main/resources/ 下建设 /META-INF/services 目录，新增一个以接口命名的文件
4、接口命名的文件填入如下内容
5、加载服务类
SqlDialect sqlDialect = ExtensionLoader.getExtensionLoader(SqlDialect.class).getActivate("mysql");6、测试
@Test
public void testSpi(){SqlDialect sqlDialect = ExtensionLoader.getExtensionLoader(SqlDialect.class).getActivate("mysql");
Assert.assertEquals("mysql",sqlDialect.dialect());
}
总结
如果有用过 dubbo 的 spi 的敌人，就会发现下面实现的思路基本上就是 dubbo 的 spi 简化版。如果是有理解过 shenyu 网关的 spi 机制的敌人，就会发现下面的实现思路和 shenyu 网关基本上是一样了。
demo 链接
https://github.com/lyb-geek/springboot-learning/tree/master/springboot-spi-enhance/springboot-spi-framework