Service Provider Interface
提供接口，可替换组件实现，插件扩展，服务发现机制
META-INF/services/接口全限定名文件中指定实现类,文件可以有多行每行一个实现类
ServiceLoader源码实现
ServiceLoader设计成一个迭代结构，即可以加载同一接口的多个实现
- 调用load只是构建了迭代器，并且延迟加载，真正遍历时才加载
- 加载到类名字符串后采取forName获取类，反射newInstance获取实例
- 缓存机制，加载过一次就把实例缓存起来，再次加载直接读缓存
ServiceLoader.java1
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| public final class ServiceLoader<S> implements Iterable<S> {
private static final String PREFIX = "META-INF/services/";
private final Class<S> service;
private final ClassLoader loader;
private final AccessControlContext acc;
private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
private LazyIterator lookupIterator;
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();
lookupIterator = new LazyIterator(service, loader);
}
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();
}
};
}
}
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查找细节
ServiceLoader.java1
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| private class LazyIterator implements Iterator<S> {
Class<S> service;
Enumeration<URL> configs = null;
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");
}
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();
}
}
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SPI应用
JDBC
DriverManager.java1
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| public class DriverManager {
static {
loadInitialDrivers();
println("JDBC DriverManager initialized");
}
private static void loadInitialDrivers() {
String drivers;
try {
drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
public String run() {
return System.getProperty("jdbc.drivers");
}
});
} catch (Exception ex) {
drivers = null;
}
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
Iterator<Driver> driversIterator = loadedDrivers.iterator();
try{
while(driversIterator.hasNext()) {
driversIterator.next();
}
} catch(Throwable t) {
}
return null;
}
});
String[] driversList = drivers.split(":");
println("number of Drivers:" + driversList.length);
for (String aDriver : driversList) {
try {
println("DriverManager.Initialize: loading " + aDriver);
Class.forName(aDriver, true, ClassLoader.getSystemClassLoader());
} catch (Exception ex) {
println("DriverManager.Initialize: load failed: " + ex);
}
}
}
}
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Mysql的驱动器包为mysql-connector-java.jar
内部包含META-INF/services/java.sql.Driver文件
文件内容为com.mysql.cj.jdbc.Driver
SPI机制进行类加载，会触发驱动类下静态初始化块，把驱动注册到JDBC的DriverManager中
Driver.java1
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| public class Driver extends NonRegisteringDriver implements java.sql.Driver {
static {
try {
java.sql.DriverManager.registerDriver(new Driver());
} catch (SQLException E) {
throw new RuntimeException("Can't register driver!");
}
}
public Driver() throws SQLException {
}
}
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整体流程
- JDBC加载DriverManager
- DriverManager静态初始化块寻找Driver触发SPI
- SPI加载到Mysql的Driver类
- Driver静态初始化块内创建实例，注册到DriverManager
