一.ServiceLoader介紹
- 一個簡單的加載服務提供者的設施。
- 系統分為兩個角色：應用程式，服務提供者jar。在應用程式中通過ServiceLoader加載所需服務。
二.使用
- jar裡面包含服務程式。
- jar META-INF檔案夾下建立services檔案夾，在services檔案裡建立檔案，檔案名為提供服務的接口的全限定名，檔案内容為實作接口的類的全限定名，多個實作類時以行分割。見圖。
- 應用程式調用服務：
ServiceLoader<NameServiceDescriptor> serviceLoader= ServiceLoader.load(NameServiceDescriptor.class); - ServiceLoader實作了Iterator接口，疊代 ServiceLoader即可得到每個NameServiceDescriptor。
三.源碼
import java.net.URL;
import java.util.ServiceConfigurationError;
public final class ServiceLoader<S> implements Iterable<S> {
private static final String PREFIX = "META-INF/services/";
//一個類或者接口對于一個ServiceLoader
private Class<S> service;
//加載服務服務提供者的ClassLoader
private ClassLoader loader;
//緩存服務提供者，按服務提供者執行個體化順序
private LinkedHashMap<String, S> providers = new LinkedHashMap<String, S>();
//ServiceLoader的疊代委托給了lookupIterator
private LazyIterator lookupIterator;
private ServiceLoader(Class<S> svc, ClassLoader cl) {
service = svc;
loader = cl;
reload();
}
public void reload() {
providers.clear();
lookupIterator = new LazyIterator(service, loader);
}
/**
* 傳回URL對應的檔案内容的Iterator，内容以行分割
* @param service 類名，傳進去隻為了異常說明更詳細
* @param u 檔案名對應的URL
*/
private Iterator<String> parse(Class service, URL u)
throws ServiceConfigurationError {
InputStream in = null;
BufferedReader r = null;
ArrayList<String> names = new ArrayList<String>();
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();
}
//從檔案裡面一行一行讀，放到names裡面
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);
}
//如果該服務提供者未被執行個體化，并且names不包含
if (!providers.containsKey(ln) && !names.contains(ln))
names.add(ln);
}
return lc + 1;
}
private static void fail(Class service, String msg, ...)throws ServiceConfigurationError{
throw new ServiceConfigurationError(service.getName() + ": " + msg);
}
private class LazyIterator implements Iterator<S> {
Class<S> service;
ClassLoader loader;
Enumeration<URL> configs = null;//檔案對應路徑URL
Iterator<String> pending = null;//檔案内容，一行一行存放
String nextName = null;//目前調用hashNext方法時，就得到下一個檔案内容。
private LazyIterator(Class<S> service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}
public boolean hasNext() {
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());
}
//儲存下一個檔案内容
nextName = pending.next();
return true;
}
public S next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
String cn = nextName;
nextName = null;
try {
//反射，關鍵所在
S p = service.cast(Class.forName(cn, true, loader)
.newInstance());
//緩存已經執行個體化的服務提供者
providers.put(cn, p);
return p;
} catch (ClassNotFoundException x) {
fail(service, "Provider " + cn + " not found");
} catch (Throwable x) {
fail(service, "Provider " + cn + " could not be instantiated: "
+ x, x);
}
throw new Error(); // This cannot happen
}
public void remove() {
throw new UnsupportedOperationException();
}
}
//以延遲方式加載服務提供者
//首先疊代被緩存的服務提供者，然後以延遲方式加載和執行個體化所有剩餘的服務提供者，依次将每個服務提供者添加到緩存。
public Iterator<S> iterator() {
return new Iterator<S>() {
//緩存起來的服務提供者
Iterator<Map.Entry<String, S>> knownProviders = providers
.entrySet().iterator();
//先去緩存服務提供者裡面找
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();
}
};
}
//執行個體化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<S>(service, loader);
}
//用目前類加載器的父加載器
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 String toString() {
return "java.util.ServiceLoader[" + service.getName() + "]";
}
}
四.Lookup
NetBeans使用的東西。和ServiceLoader提供一樣的功能。具體參見http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/lookup/doc-files/lookup-api.html
五.為什麼要使用
- 為什麼不直接使用反射，得到服務提供者。因為ServiceLoader提供了緩存機制，因為Lookup提供了監聽機制。還有沒有其他原因？
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