SPI是什么
SPI全称Service Provider Interface,是Java提供的一套用来被第三方实现或者扩展的API,它可以用来启用框架扩展和替换组件
Java SPI 实际上是“基于接口的编程+策略模式+配置文件”组合实现的动态加载机制
简单使用
新建一个Android项目,和三个module工程(interface,adisplay,bdisplay)
项目的依赖关系
在interfaceModel中新建一个接口
public interface Display {
String display();
}
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在adisplay模块中实现这个接口
public class ADisplay implements Display {
@Override
public String display() {
return "This is display in module adisplay";
}
}
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在adisplay模块中新建一个文件
注意这个两个地方的路径,文本内容是当前module中实现类的全路径
bdisplay模块页仿照adisplay操作一遍
app中定一个DisplayImpl的实现
public class DisplayImpl implements Display {
@Override
public String display() {
return "This is display in module app";
}
}
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仿照adisplay新建配置文件。 在页面上添加一个按钮来实现
val loader: ServiceLoader<Display> = ServiceLoader.load(Display::class.java)
var mIterator = loader.iterator()
while (mIterator.hasNext()) {
mIterator.next().display()
}
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通过ServiceLoader来加载接口的不同实现类,然后会得到迭代器,在迭代器中可以拿到不同实现类全限定名,然后通过反射动态加载实例就可以调用display方法了
源码追踪
主要起作用的是ServiceLoader这个类
public final class ServiceLoader<S>
implements Iterable<S>
{
//配置文件所在的包目录路径
private static final String PREFIX = "META-INF/services/";
private final Class<S> service;//接口名称
private final ClassLoader loader;//类加载器
//providers就是不同实现类的缓存,key就是实现类的全限定名,value就是实现类的实例
private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
private LazyIterator lookupIterator;//内部类LazyIterator的实例
public void reload() {
providers.clear();
lookupIterator = new LazyIterator(service, loader);
}
private ServiceLoader(Class<S> svc, ClassLoader cl) {
service = Objects.requireNonNull(svc, "Service interface cannot be null");
loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
reload();
}
private static void fail(Class<?> service, String msg, Throwable cause)
throws ServiceConfigurationError
{
throw new ServiceConfigurationError(service.getName() + ": " + msg,
cause);
}
private static void fail(Class<?> service, String msg)
throws ServiceConfigurationError
{
throw new ServiceConfigurationError(service.getName() + ": " + msg);
}
private static void fail(Class<?> service, URL u, int line, String msg)
throws ServiceConfigurationError
{
fail(service, u + ":" + line + ": " + msg);
}
private int parseLine(Class<?> service, URL u, BufferedReader r, int lc,
List<String> names)
throws IOException, ServiceConfigurationError
{
String ln = r.readLine();
if (ln == null) {
return -1;
}
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);
}
if (!providers.containsKey(ln) && !names.contains(ln))
names.add(ln);
}
return lc + 1;
}
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;
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 class LazyIterator
implements Iterator<S>
{
Class<S> service;
ClassLoader loader;
Enumeration<URL> configs = null;
Iterator<String> pending = null;
String nextName = null;
private LazyIterator(Class<S> service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}
private boolean hasNextService() {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
String fullName = PREFIX + service.getName();
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);
} catch (IOException x) {
fail(service, "Error locating configuration files", x);
}
}
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
pending = parse(service, configs.nextElement());
}
nextName = pending.next();
return true;
}
private S nextService() {
if (!hasNextService())
throw new NoSuchElementException();
String cn = nextName;
nextName = null;
Class<?> c = null;
try {
c = Class.forName(cn, false, loader);
} catch (ClassNotFoundException x) {
fail(service,
"Provider " + cn + " not found", x);
}
if (!service.isAssignableFrom(c)) {
ClassCastException cce = new ClassCastException(
service.getCanonicalName() + " is not assignable from " + c.getCanonicalName());
fail(service,
"Provider " + cn + " not a subtype", cce);
}
try {
S p = service.cast(c.newInstance());
providers.put(cn, p);
return p;
} catch (Throwable x) {
fail(service,
"Provider " + cn + " could not be instantiated",
x);
}
throw new Error(); // This cannot happen
}
public boolean hasNext() {
return hasNextService();
}
public S next() {
return nextService();
}
public void remove() {
throw new UnsupportedOperationException();
}
}
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();
}
};
}
public static <S> ServiceLoader<S> load(Class<S> service,
ClassLoader loader)
{
return new ServiceLoader<>(service, loader);
}
public static <S> ServiceLoader<S> load(Class<S> service) {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return ServiceLoader.load(service, cl);
}
public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
ClassLoader cl = ClassLoader.getSystemClassLoader();
ClassLoader prev = null;
while (cl != null) {
prev = cl;
cl = cl.getParent();
}
return ServiceLoader.load(service, prev);
}
public static <S> S loadFromSystemProperty(final Class<S> service) {
try {
final String className = System.getProperty(service.getName());
if (className != null) {
Class<?> c = ClassLoader.getSystemClassLoader().loadClass(className);
return (S) c.newInstance();
}
return null;
} catch (Exception e) {
throw new Error(e);
}
}
public String toString() {
return "java.util.ServiceLoader[" + service.getName() + "]";
}
}
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ServiceLoader提供了两个静态的load方法,如果我们没有传入类加载器,ServiceLoader会自动为我们获得一个当前线程的类加载器,最终都是调用构造函数。
public void reload() {
providers.clear();
lookupIterator = new LazyIterator(service, loader);
}
private ServiceLoader(Class<S> svc, ClassLoader cl) {
service = Objects.requireNonNull(svc, "Service interface cannot be null");
loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
reload();
}
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构造函数中工作很简单就是清除实现类的缓存,实例化迭代器
loader.iterator() 返回一个迭代器。首先会到providers中去查找有没有存在的实例,有就直接返回,没有再到LazyIterator中查找
private boolean hasNextService() {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
String fullName = PREFIX + service.getName();
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);
} catch (IOException x) {
fail(service, "Error locating configuration files", x);
}
}
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
pending = parse(service, configs.nextElement());
}
nextName = pending.next();
return true;
}
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- 首先拿到配置文件名fullName
- 通过类加载器获得所有模块的配置文件
- 依次扫描每个配置文件的内容,返回配置文件内容
Iterator<String> pending
,每个配置文件中可能有多个实现类的全限定名,所以pending也是个迭代器
分析nextService方法
private S nextService() {
if (!hasNextService())
throw new NoSuchElementException();
String cn = nextName;
nextName = null;
Class<?> c = null;
try {
c = Class.forName(cn, false, loader);
} catch (ClassNotFoundException x) {
fail(service,
"Provider " + cn + " not found", x);
}
if (!service.isAssignableFrom(c)) {
ClassCastException cce = new ClassCastException(
service.getCanonicalName() + " is not assignable from " + c.getCanonicalName());
fail(service,
"Provider " + cn + " not a subtype", cce);
}
try {
S p = service.cast(c.newInstance());
providers.put(cn, p);
return p;
} catch (Throwable x) {
fail(service,
"Provider " + cn + " could not be instantiated",
x);
}
throw new Error(); // This cannot happen
}
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- 首先根据nextName,Class.forName加载拿到具体实现类的class对象
- Class.newInstance()实例化拿到具体实现类的实例对象
- 将实例对象转换service.cast为接口
- 将实例对象放到缓存中,providers.put(cn, p),key就是实现类的全限定名,value是实例对象。
- 返回实例对象
小结
-
1 应用程序调用ServiceLoader.load方法 ServiceLoader.load方法内先创建一个新的ServiceLoader,并实例化该类中的成员变量,包括:
- loader(ClassLoader类型,类加载器)
- acc(AccessControlContext类型,访问控制器)
- providers(LinkedHashMap<String,S>类型,用于缓存加载成功的类)
- lookupIterator(实现迭代器功能)
-
2 应用程序通过迭代器接口获取对象实例
ServiceLoader先判断成员变量providers对象中(LinkedHashMap<String,S>类型)是否有缓存实例对象,如果有缓存,直接返回。如果没有缓存,执行类的装载- 读取META-INF/services/下的配置文件,获得所有能被实例化的类的名称,值得注意的是,ServiceLoader可以跨越jar包获取META-INF下的配置文件
- 通过反射方法Class.forName()加载类对象,并用instance()方法将类实例化。
- 把实例化后的类缓存到providers对象中,(LinkedHashMap<String,S>类型)然后返回实例对象
总结
- 使用Java SPI机制的优势是实现解耦,使得第三方服务模块的装配控制的逻辑与调用者的业务代码分离,而不是耦合在一起。
- 相比使用提供接口jar包,供第三方服务模块实现接口的方式,SPI的方式使得源框架,不必关心接口的实现类的路径
- 第三方服务模块实现接口后,在第三方的项目代码的META-INF/services目录下的配置文件指定实现类的全路径名,源码框架即可找到实现类
- 虽然ServiceLoader也算是使用的延迟加载,但是基本只能通过遍历全部获取,也就是接口的实现类全部加载并实例化一遍。如果你并不想用某些实现类,它也被加载并实例化了,这就造成了浪费。获取某个实现类的方式不够灵活,只能通过Iterator形式获取,不能根据某个参数来获取对应的实现类。
- 多个并发多线程使用ServiceLoader类的实例是不安全的。