前言
如果用过JDK提供的SPI机制的朋友，大概就会知道它无法按需加载。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，这种方案就是要借用spring。今天我们在聊聊另外一种实现方式，就是我们自己手写一个
2
实现思路
整体思路和jdk实现spi差不多，如果对jdk实现的spi不了解，可以查看我之前写的文章java之spi机制简介。差别就是我们在配置文件是以key-value的形式存在，形如
代码语言：javascript复制springMysql=com.github.lybgeek.dialect.mysql.SpringMysqlDialect3
实现逻辑
1、约定好要进行解析的目录，比如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，比如
com.github.lybgeek.dialect.SpringSqlDialect3、约定好文件内容格式，比如
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、根据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;
}4
核心代码
代码语言：javascript复制@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;
}
}
}5
使用示例
1、定义服务接口
代码语言：javascript复制@SPI("mysql")
public interface SqlDialect {
String dialect();
}2、定义具体实现类
代码语言：javascript复制@Activate
public class MysqlDialect implements SqlDialect {
@Override
public String dialect() {
return "mysql";
}
}3、src/main/resources/下建立/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
代码语言：javascript复制SqlDialect sqlDialect = ExtensionLoader.getExtensionLoader(SqlDialect.class).getActivate("mysql");6、测试
代码语言：javascript复制@Test
public void testSpi(){
SqlDialect sqlDialect = ExtensionLoader.getExtensionLoader(SqlDialect.class).getActivate("mysql");
Assert.assertEquals("mysql",sqlDialect.dialect());
}
6
总结
如果有用过dubbo的spi的朋友，就会发现上面实现的思路基本上就是dubbo的spi简化版。如果是有了解过shenyu网关的spi机制的朋友，就会发现上面的实现思路和shenyu网关基本上是一样了。
7
demo链接
https://github.com/lyb-geek/springboot-learning/tree/master/springboot-spi-enhance/springboot-spi-framework
