Java SPI
在系统设计的时候, 实现方案里各个模块之间基于接口编程, 模块之间不对实现类进行硬编码
这句话的意思是: 接口A,可以有一个或多个实现类, 在代码中对于实现类不可以写死, 如果一旦代码中设计到了具体的实现类, 那么就违法了可插拔的原则, 所以模块之间不对实现类进行硬编码(如果有不懂可以找我讨论~)
Java SPI 实现的好处就是对于接口的实现类,实现可插拔式, 可以自己需要使用不同实现类, 解耦的概念
使用场景
什么时候使用 java spi , spi应用场景, 调用者可以根据自己的需要扩展接口, 使用不同接口, 可插拔
JDBC数据库驱动的时候使用了不同类型的数据, SPI方式实现
Dubbo , 不过dubbo进行了java spi再次封装
具体使用
- 必须在resource下面创建META-INF/Services 目录下面创建一个接口全包名+类名的文件
- 文件里面的内容是实现类的全报名+类名
- 实现类中必须有一个不带参数的构造方法
public interface IJudy {
void xjf();
}
public class Judy1 implements IJudy{
public void xjf() {
System.out.println("Judy1");
}
}
public class Judy2 implements IJudy {
public void xjf() {
System.out.println("Judy2");
}
}
com.judy.spi.Judy1
com.judy.spi.Judy2
public class JudyMain {
public static void main(String[] args){
ServiceLoader<IJudy> judies = ServiceLoader.load(IJudy.class);
for (IJudy judy : judies) {
judy.xjf();
}
}
}
打印出结果
Judy1
Judy2
Dubbo SPI
dubbo SPI 是在Java SPI 上进行扩展, 符合以下规范就可以
- 必须在MATE-INF/dubbo . META-INF/dubbo/internal/
- dubbo使用的是ExtensionLoader扩展容器
优先级
1 在类上加@Adaptive 优先级最高
2 @SPI(“Value”)
3从URl中获取
4在方法上加@Adaptive,
应用
1类上@Adaptive
@Adaptive
public class ThriftAdaptiveExt2 implements AdaptiveExt2 {
@Override
public String echo(String msg, URL url) {
return "thrift";
}
}
public class Test2 {
public static void main(String[] args){
ExtensionLoader<AdaptiveExt2> loader = ExtensionLoader.getExtensionLoader(AdaptiveExt2.class);
AdaptiveExt2 adaptiveExtension = loader.getAdaptiveExtension();
URL url = URL.valueOf("test://localhost/test");
System.out.println(adaptiveExtension.echo("c",url));
}
}
运行结果:thrift
2 方法,从路径获取key
方法上加@Adaptive在url上指定使用哪个实现
public interface AdaptiveExt2 {
@Adaptive({"t"})
String echo(String msg, URL url);
}
public class Test2 {
public static void main(String[] args){
ExtensionLoader<AdaptiveExt2> loader = ExtensionLoader.getExtensionLoader(AdaptiveExt2.class);
AdaptiveExt2 adaptiveExtension = loader.getAdaptiveExtension();
URL url = URL.valueOf("test://localhost/test?t=cloud");
System.out.println(adaptiveExtension.echo("c",url));
}
}
spring cloud
3 是@SPI
@SPI("cloud")
public interface AdaptiveExt2 {
@Adaptive
String echo(String msg, URL url);
}
public class DubboAdaptiveExt2 implements AdaptiveExt2 {
@Override
public String echo(String msg, URL url) {
return "dubbo";
}
}
public class SpringCloudAdaptiveExt2 implements AdaptiveExt2 {
@Override
public String echo(String msg, URL url) {
return "spring cloud";
}
}
public class Test {
public static void main(String[] args) {
ExtensionLoader<AdaptiveExt2> loader = ExtensionLoader.getExtensionLoader(AdaptiveExt2.class);
AdaptiveExt2 adaptiveExtension = loader.getAdaptiveExtension();
URL url = URL.valueOf("test://localhost:/test");
System.out.println(adaptiveExtension.echo("c",url));
}
}
dubbo=com.mooc.jiangzh.dubbo.SPIDubbo.DubboAdaptiveExt2
cloud=com.mooc.jiangzh.dubbo.SPIDubbo.SpringCloudAdaptiveExt2
thrift=com.mooc.jiangzh.dubbo.SPIDubbo.ThriftAdaptiveExt2
运行救过 spring cloud
为什么是spring cloud ? 首先我们定义一个接口, 并且实现这个接口的有三个类, 他是如何知道具体使用哪个实现类的, 原因是因为 @SPI(cloud)
源码分析
从 ExtensionLoader.getExtensionLoader(AdaptiveExt2.class) 入手
/**
* 得到使用什么接口
*/
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
/**
* 条件判断
*/
if (type == null) {
throw new IllegalArgumentException("Extension type == null");
}
if (!type.isInterface()) {
throw new IllegalArgumentException("Extension type (" + type + ") is not an interface!");
}
// 这个类上面必须使用@SPI注解否则报错
if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type (" + type +
") is not an extension, because it is NOT annotated with @" + SPI.class.getSimpleName() + "!");
}
}
/**
* 首先判断一下是否存在, 如果存在则直接返回
*/
ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
if (loader == null) {
// 放到EXTENSION_LOADERS , 生成ExtensionLoader 对象
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
}
return loader;
}
再从 AdaptiveExt2 adaptiveExtension = loader.getAdaptiveExtension(); 出发
public T getAdaptiveExtension() {
//获取一个实例
Object instance = cachedAdaptiveInstance.get();
/**
* 这里使用了双重锁的概念, 不会的可以看单利模式的双重锁
*/
if (instance == null) {
if (createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
//第一次肯定是空实例,所以创建一个适配扩展实例
instance = createAdaptiveExtension();
//为了缓存使用,instance实例化之后存储起来
cachedAdaptiveInstance.set(instance);
} catch (Throwable t) {
createAdaptiveInstanceError = t;
throw new IllegalStateException("Failed to create adaptive instance: " + t.toString(), t);
}
}
}
} else {
throw new IllegalStateException("Failed to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
}
}
return (T) instance;
}
/**
* 获取到适配器类的class, 然后利用反射创建适配器类的实例
*/
private T createAdaptiveExtension() {
try {
//根据规则找到类,然后实例化
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can't create adaptive extension " + type + ", cause: " + e.getMessage(), e);
}
}
private Class<?> getAdaptiveExtensionClass() {
//扫描@SPI
getExtensionClasses();
//cachedAdaptiveClass是从getExtensionClasses里过来的,主要的作用就是根据规则找到路径下的类文件
// 获取cachedAdaptiveClass, cachedAdaptiveClass他的作用是判断是否为自适应扩展点. 是否加了注解@
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
//根据反射创建类 AdaptiveClass
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
/**
* 加载扩展点的实现类
* @return
*/
private Map<String, Class<?>> getExtensionClasses() {
Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
//加载这个扩展类在那个路径下面
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
// 此方法已经getExtensionClasses方法同步过。
private Map<String, Class<?>> loadExtensionClasses() {
// 获取类上面是否有@SPI注解
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if(defaultAnnotation != null) {
String value = defaultAnnotation.value();
if(value != null && (value = value.trim()).length() > 0) {
String[] names = NAME_SEPARATOR.split(value);
if(names.length > 1) {
throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
+ ": " + Arrays.toString(names));
}
//如果有注解,把value中的值赋给cachedDefaultName
if(names.length == 1) cachedDefaultName = names[0];
}
}
/**
*1 从MEAT-INF/dubbo/internal/
2从 MEAT-INF/dubbo
3从MEAT-INF/services/
获取权限定名称
*
*/
Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
loadFile(extensionClasses, DUBBO_DIRECTORY);
loadFile(extensionClasses, SERVICES_DIRECTORY);
return extensionClasses;
}
private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
String fileName = dir + type.getName();
try {
Enumeration<java.net.URL> urls;
ClassLoader classLoader = findClassLoader();
if (classLoader != null) {
urls = classLoader.getResources(fileName);
} else {
urls = ClassLoader.getSystemResources(fileName);
}
if (urls != null) {
while (urls.hasMoreElements()) {
java.net.URL url = urls.nextElement();
try {
BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
try {
String line = null;
while ((line = reader.readLine()) != null) {
final int ci = line.indexOf('#');
if (ci >= 0) line = line.substring(0, ci);
line = line.trim();
if (line.length() > 0) {
try {
String name = null;
// 后去全路径名
int i = line.indexOf('=');
if (i > 0) {
name = line.substring(0, i).trim();
line = line.substring(i + 1).trim();
}
if (line.length() > 0) {
// 判断是新加载clazz是否是type的子类
Class<?> clazz = Class.forName(line, true, classLoader);
if (! type.isAssignableFrom(clazz)) {
throw new IllegalStateException("Error when load extension class(interface: " +
type + ", class line: " + clazz.getName() + "), class "
+ clazz.getName() + "is not subtype of interface.");
}
//判断在这个类上是否有Adaptive的注解
if (clazz.isAnnotationPresent(Adaptive.class)) {
if(cachedAdaptiveClass == null) {
//存储
cachedAdaptiveClass = clazz;
//如果有多个Adaptive类的实例则报错
} else if (! cachedAdaptiveClass.equals(clazz)) {
throw new IllegalStateException("More than 1 adaptive class found: "
+ cachedAdaptiveClass.getClass().getName()
+ ", " + clazz.getClass().getName());
}
} else {
try {
clazz.getConstructor(type);
Set<Class<?>> wrappers = cachedWrapperClasses;
if (wrappers == null) {
cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
wrappers = cachedWrapperClasses;
}
wrappers.add(clazz);
} catch (NoSuchMethodException e) {
clazz.getConstructor();
if (name == null || name.length() == 0) {
name = findAnnotationName(clazz);
if (name == null || name.length() == 0) {
if (clazz.getSimpleName().length() > type.getSimpleName().length()
&& clazz.getSimpleName().endsWith(type.getSimpleName())) {
name = clazz.getSimpleName().substring(0, clazz.getSimpleName().length() - type.getSimpleName().length()).toLowerCase();
} else {
throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + url);
}
}
}
String[] names = NAME_SEPARATOR.split(name);
if (names != null && names.length > 0) {
Activate activate = clazz.getAnnotation(Activate.class);
if (activate != null) {
cachedActivates.put(names[0], activate);
}
for (String n : names) {
if (! cachedNames.containsKey(clazz)) {
cachedNames.put(clazz, n);
}
Class<?> c = extensionClasses.get(n);
if (c == null) {
extensionClasses.put(n, clazz);
} else if (c != clazz) {
throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
}
}
}
}
}
}
} catch (Throwable t) {
IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + url + ", cause: " + t.getMessage(), t);
exceptions.put(line, e);
}
}
} // end of while read lines
} finally {
reader.close();
}
} catch (Throwable t) {
logger.error("Exception when load extension class(interface: " +
type + ", class file: " + url + ") in " + url, t);
}
} // end of while urls
}
} catch (Throwable t) {
logger.error("Exception when load extension class(interface: " +
type + ", description file: " + fileName + ").", t);
}
}
刚才上面只是对 getExtensionClasses();做了解释,下面针对的是createAdaptiveExtensionClass();,如果cachedAdaptiveClass 为空,也就是没有@Adaptive注解的时候,怎么办?
private Class<?> getAdaptiveExtensionClass() {
getExtensionClasses();
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
下面主要是针对的是URl进行操作的了
private Class<?> createAdaptiveExtensionClass() {
// 创建代码的字符串形式
String code = createAdaptiveExtensionClassCode();
//寻找类加载器
ClassLoader classLoader = findClassLoader();
//找到适配器扩展类
com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
//进行编译成class的实例返回
return compiler.compile(code, classLoader);
}