前言
如果用過JDK提供的SPI機制的朋友，大概就會知道它無法按需載入。之前寫過一篇文章聊聊基於jdk實現的spi如何與spring整合實現依賴注入。利用spring的依賴注入來實現spi按需載入，這種方案就是要借用spring。今天我們在聊聊另外一種實現方式，就是我們自己手寫一個
實現思路
整體思路和jdk實現spi差不多，如果對jdk實現的spi不瞭解，可以檢視我之前寫的文章java之spi機制簡介。差別就是我們在配置檔案是以key-value的形式存在，形如
springMysql=com.github.lybgeek.dialect.mysql.SpringMysqlDialect實現邏輯
1、約定好要進行解析的目錄，比如META-INF/services/
private static final String SERVICE_DIRECTORY = "META-INF/services/";2、約定好要解析的檔名命名，比如
com.github.lybgeek.dialect.SpringSqlDialect3、約定好檔案內容格式，比如
springMysql=com.github.lybgeek.dialect.mysql.SpringMysqlDialect4、獲取約定好的目錄，解析檔案，並將相應內容放入快取
/**
* 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、根據key，去快取查詢相應的類例項
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;
}核心程式碼
@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;
}
}
}
使用示例
1、定義服務介面
@SPI("mysql")
public interface SqlDialect {
String dialect();
}
2、定義具體實現類
@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/下建立/META-INF/services 目錄，新增一個以介面命名的檔案
4、介面命名的檔案填入如下內容
5、載入服務類
SqlDialect sqlDialect = ExtensionLoader.getExtensionLoader(SqlDialect.class).getActivate("mysql");6、測試
@Test
public void testSpi(){
SqlDialect sqlDialect = ExtensionLoader.getExtensionLoader(SqlDialect.class).getActivate("mysql");
Assert.assertEquals("mysql",sqlDialect.dialect());
}
總結
如果有用過dubbo的spi的朋友，就會發現上面實現的思路基本上就是dubbo的spi簡化版。如果是有了解過shenyu閘道器的spi機制的朋友，就會發現上面的實現思路和shenyu閘道器基本上是一樣了。