Dubbo SPI(一)
在讨论Dubbo spi之前 首先了解一下Java spi机制
什么是Java的spi
SPI(Service Provider Interface)是一种服务发现机制。SPI的本质是将接口实现类的全限定名写在一个文件中,由加载器去加载这些接口实现类,这样可以在运行时动态将接口替换为实现类,通过SPI机制可以拓展功能,且SPI机制在框架中也有所应用。
Java spi示例
首先定义一个接口:
public interface Hello {
void say();
}
为该接口添加两个实现类:
public class HelloImpl1 implements Hello {
@Override
public void say() {
System.out.println("我是实现类1");
}
}
public class HelloImpl2 implements Hello {
@Override
public void say() {
System.out.println("我是实现类2");
}
}
在resource的META-INF下面创建文件夹services,并创建名称为接口全限定名的文件:
com.lgx.study.spi.Hello文件
文件内容为:
com.lgx.study.spi.HelloImpl1
com.lgx.study.spi.HelloImpl2
来测试一下:
public class Test {
public static void main(String[] args) {
ServiceLoader<Hello> loader = ServiceLoader.load(Hello.class);
Iterator<Hello> iterator = loader.iterator();
while (iterator.hasNext()){
Hello next = iterator.next();
next.say();
}
}
}
运行结果:
我是实现类1
我是实现类2
我们从源码进行简单分析:
ServiceLoader
#ServiceLoader
public static <S> ServiceLoader<S> load(Class<S> service) {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return ServiceLoader.load(service, cl);
}
public static <S> ServiceLoader<S> load(Class<S> service,
ClassLoader loader)
{
return new ServiceLoader<>(service, loader);
}
private ServiceLoader(Class<S> svc, ClassLoader cl) {
service = Objects.requireNonNull(svc, "Service interface cannot be null");
loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
reload();
}
public void reload() {
providers.clear();
// LazyIterator是ServiceLoader的内部类实现了Iterator接口
lookupIterator = new LazyIterator(service, loader);
}
可以看到一段代码比较简单就是创建了一个ServiceLoader,并将类加载器和接口的Class类传入进行实例化,并创建了一个迭代器
再看 这一行 : Iterator
#ServiceLoader
public Iterator<S> iterator() {
return new Iterator<S>() {
Iterator<Map.Entry<String,S>> knownProviders
= providers.entrySet().iterator();
public boolean hasNext() {
if (knownProviders.hasNext())
return true;
return lookupIterator.hasNext();
}
public S next() {
if (knownProviders.hasNext())
return knownProviders.next().getValue();
return lookupIterator.next();
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
该行代码的作用可以看出,是创建一个迭代器,初始化knownProviders
接下来进入while循环中:
iterator.hasNext():
public boolean hasNext() {
if (knownProviders.hasNext())
return true;
// 进入到这一步中
return lookupIterator.hasNext();
}
public boolean hasNext() {
if (acc == null) {
// 进入到这里
return hasNextService();
} else {
PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {
public Boolean run() { return hasNextService(); }
};
return AccessController.doPrivileged(action, acc);
}
}
private boolean hasNextService() {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
// PREFIX为:META-INF/services
// service.getName()为:com.lgx.study.spi.Hello
// 这里也就解释了为什么我们要在META-INF/services下面创建名称为接口
// 全限定名的文件
String fullName = PREFIX + service.getName();
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
// 从resource下加载该文件
configs = loader.getResources(fullName);
} catch (IOException x) {
fail(service, "Error locating configuration files", x);
}
}
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
// 来到这里
pending = parse(service, configs.nextElement());
}
nextName = pending.next();
return true;
}
private Iterator<String> parse(Class<?> service, URL u)
throws ServiceConfigurationError
{
InputStream in = null;
BufferedReader r = null;
ArrayList<String> names = new ArrayList<>();
try {
in = u.openStream();
r = new BufferedReader(new InputStreamReader(in, "utf-8"));
int lc = 1;
// 在这里做文件解析 继续往下跟
while ((lc = parseLine(service, u, r, lc, names)) >= 0);
} catch (IOException x) {
fail(service, "Error reading configuration file", x);
} finally {
try {
if (r != null) r.close();
if (in != null) in.close();
} catch (IOException y) {
fail(service, "Error closing configuration file", y);
}
}
return names.iterator();
}
private int parseLine(Class<?> service, URL u, BufferedReader r, int lc,
List<String> names)
throws IOException, ServiceConfigurationError
{
......
// 以下为具体的解析过程
int ci = ln.indexOf('#');
if (ci >= 0) ln = ln.substring(0, ci);
ln = ln.trim();
int n = ln.length();
if (n != 0) {
if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
fail(service, u, lc, "Illegal configuration-file syntax");
int cp = ln.codePointAt(0);
if (!Character.isJavaIdentifierStart(cp))
fail(service, u, lc, "Illegal provider-class name: " + ln);
for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
cp = ln.codePointAt(i);
if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
fail(service, u, lc, "Illegal provider-class name: " + ln);
}
// 解析完的ln为com.lgx.study.spi.HelloImpl1和com.lgx.study.spi.HelloImpl2
// 将解析出来的实现类的权限名加入到List中
if (!providers.containsKey(ln) && !names.contains(ln))
names.add(ln);
}
return lc + 1;
}
解析完了返回Iterator
接着是调用 iterator.next()拿到接口:
public S next() {
if (knownProviders.hasNext())
return knownProviders.next().getValue();
// 来到这里 继续往下走
return lookupIterator.next();
}
public S next() {
if (acc == null) {
// 继续走
return nextService();
}
......
}
private S nextService() {
if (!hasNextService())
throw new NoSuchElementException();
// 这里的cn为实现类的全权限名:com.lgx.study.spi.HelloImpl1和com.lgx.study.spi.HelloImpl2
String cn = nextName;
nextName = null;
Class<?> c = null;
try {
// 通过类加载器加载该类
c = Class.forName(cn, false, loader);
} catch (ClassNotFoundException x) {
fail(service,
"Provider " + cn + " not found");
}
if (!service.isAssignableFrom(c)) {
fail(service,
"Provider " + cn + " not a subtype");
}
try {
// 创建该类的实例
S p = service.cast(c.newInstance());
// 加入到map中缓存:
// map<com.lgx.study.spi.HelloImpl1, HelloImpl1实例>
// map<com.lgx.study.spi.HelloImpl2, HelloImpl2实例>
providers.put(cn, p);
return p;
} catch (Throwable x) {
fail(service,
"Provider " + cn + " could not be instantiated",
x);
}
throw new Error(); // This cannot happen
}
到这里所有的代码都看完了,其实看起来也很简单:
通过加载META-INFO/services下接口文件,解析文件内的每一行内容放入集合中,再遍历这个集合,拿到实现类的权限定名,并使用类加载器进行加载,最后创建该类的实例加入到缓存中,最后返回改实例。