Serviceloader探险
源码
Java中的SPI机制是服务定义一个接口com.daltao.X，之后服务提供方提供一个实现类，包含在某个jar包下。对应的jar包的META-INF/services/下需要放入一个文件com.daltao.X，其中每一行写入一个实现类。这样服务的使用方就可以通过这种方式查找到服务的实现类并加载。
ServiceLoader是Java中用于实现SPI机制的一个组件，其用于遍历某个接口对应的所有的配置文件中的配置项。比如java中的Charset类就使用了ServiceLoader。
public abstract class Charset
implements Comparable<Charset>
public static SortedMap<String,Charset> availableCharsets() {
return AccessController.doPrivileged(
new PrivilegedAction<SortedMap<String,Charset>>() {
public SortedMap<String,Charset> run() {
TreeMap<String,Charset> m =
new TreeMap<String,Charset>(
ASCIICaseInsensitiveComparator.CASE_INSENSITIVE_ORDER);
put(standardProvider.charsets(), m);
CharsetProvider ecp = ExtendedProviderHolder.extendedProvider;
if (ecp != null)
put(ecp.charsets(), m);
for (Iterator<CharsetProvider> i = providers(); i.hasNext();) {
CharsetProvider cp = i.next();
put(cp.charsets(), m);
}
return Collections.unmodifiableSortedMap(m);
}
});
}
private static Iterator<CharsetProvider> providers() {
return new Iterator<CharsetProvider>() {
ClassLoader cl = ClassLoader.getSystemClassLoader();
//利用系统ClassLoader去查找所有的CharsetProvider的服务提供方
ServiceLoader<CharsetProvider> sl =
ServiceLoader.load(CharsetProvider.class, cl);
Iterator<CharsetProvider> i = sl.iterator();
CharsetProvider next = null;
private boolean getNext() {
while (next == null) {
try {
if (!i.hasNext())
return false;
next = i.next();
} catch (ServiceConfigurationError sce) {
if (sce.getCause() instanceof SecurityException) {
// Ignore security exceptions
continue;
}
throw sce;
}
}
return true;
}
public boolean hasNext() {
return getNext();
}
public CharsetProvider next() {
if (!getNext())
throw new NoSuchElementException();
CharsetProvider n = next;
next = null;
return n;
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
}
Charset类实际上也是通过ServiceLoader来查找所有的服务提供方的。下面就讲讲ServiceLoader的实现。
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);
}
这两个都是比较常用的工厂方法，实际上都转发给了ServiceLoader的构造器。接下来看看构造器的内容。
public final class ServiceLoader<S>
implements Iterable<S>
{
private static final String PREFIX = "META-INF/services/";
// The class or interface representing the service being loaded
private final Class<S> service;
// The class loader used to locate, load, and instantiate providers
private final ClassLoader loader;
// The access control context taken when the ServiceLoader is created
private final AccessControlContext acc;
//可以理解provider是一个全局的排重缓存
// Cached providers, in instantiation order
private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
//惰性的迭代器
// The current lazy-lookup iterator
private LazyIterator lookupIterator;
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;
//这里会做准备操作
reload();
}
public void reload() {
providers.clear();
lookupIterator = new LazyIterator(service, loader);
}
//ServiceLoader的主要用处，返回一个迭代器，可以遍历所有的服务提供者
//这里优先找providers缓存中的内容，如果没有才去调用内部的迭代器，找下一个文件
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() {
//如果遍历完了上一次的配置文件，就使用下一个配置文件
if (knownProviders.hasNext())
return knownProviders.next().getValue();
return lookupIterator.next();
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
}
很显然其中的看头在于LazyIterator的实现类。LazyIterator是ServiceLoader的一个内部类，其用于遍历所有的配置文件。
// Private inner class implementing fully-lazy provider lookup
//
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;
}
//判断是否还有下一个配置文件
private boolean hasNextService() {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
//这里名称是拼接出来的
//PREFIX的定义为：private static final String PREFIX = "META-INF/services/";
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;
//先取到下一个实现类名称，并加载这个类
try {
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 {
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() {
if (acc == null) {
return hasNextService();
} else {
PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {
public Boolean run() { return hasNextService(); }
};
return AccessController.doPrivileged(action, acc);
}
}
public S next() {
if (acc == null) {
return nextService();
} else {
PrivilegedAction<S> action = new PrivilegedAction<S>() {
public S run() { return nextService(); }
};
return AccessController.doPrivileged(action, acc);
}
}
public void remove() {
throw new UnsupportedOperationException();
}
}
其中parse的方式非常粗犷。
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();
}
////每次读取一行，如果有#号，则截取#号前面的内容
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);
}
//将parse到的行加入到names中
if (!providers.containsKey(ln) && !names.contains(ln))
names.add(ln);
}
return lc + 1;
}