Java SPI的简单小实例

Java SPI的简单小实例

JDK有个ServiceLoader类，在java.util包里，支持按约定目录/META-INF/services去找到接口全路径命名的文件，读取文件内容得到接口实现类的全路径，加载并实例化。如果我们在自己的代码中定义一个接口，别人按接口实现并打包好了，那么我们只需要引入jar包，通过ServiceLoader就能够把别人的实现用起来。举个例子，JDK中的JDBC提供一个数据库连接驱动接口，不同的厂商可以有不同的实现，如果它们给的jar包里按规定提供了配置和实现类，那么我们就可以执行不同的数据库连接操作，比如mysql的jar包里就会有自己的配置：

这里文件名就是接口：

文件内容是实现类：

我们自己实现一个简单例子，不需要打jar包，把目录放到spring boot的resources下即可，这里就是classpath，跟你放jar包里效果一样。

1、定义一个接口：

package com.wlf.service;

public interface ITest {

void saySomething();

}

2、定义两个实现：

package com.wlf.service.impl;

import com.wlf.service.ITest;

public class ITestImpl1 implements ITest {

@Override

public void saySomething() {

System.out.println("Hi, mia.");

}

}

package com.wlf.service.impl;

import com.wlf.service.ITest;

public class ITestImpl2 implements ITest {

@Override

public void saySomething() {

System.out.println("Hello, world.");

}

}

3、按预定新增/META-INF/services/com.wlf.service.ITest文件：

com.wlf.service.impl.ITestImpl1

com.wlf.service.impl.ITestImpl2

4、ZoavF定义一个执行类，通过ServiceLoader加载并实例化，调用实现类方法，跑一下：

package com.wlf.service;

import java.util.Iterator;

import java.util.ServiceLoader;

public class TestServiceLoader {

public static void main(String[] args) {

ServiceLoader serviceLoader = ServiceLoader.load(ITest.class);

Iterator iTests = serviceLoader.iterator();

while (iTests.hasNext()) {

ITest iTest = iTests.next();

System.out.printf("loading %s\n", iTest.getClass().getName());

iTest.saySomething();

}

}

}

打印结果：

ServiceLoader源码比较简单，可以看下上面我们使用到的标黄了的方法：

/**

* Lazily loads the available providers of this loader's service.

*

*

The iterator returned by this method first yields all of the

* elements of the provider cache, in instantiation order. It then lazily

* loads and instantiates any remaining providers, adding each one to the

* cache in turn.

*

*

To achieve laziness the actual work of parsing the available

* provider-configuration files and instantiating providers must be done by

* the iterator itself. Its {@link java.util.Iterator#hasNext hasNext} and

* {@link java.util.Iterator#next next} methods can therefore throw a

* {@link ServiceConfigurationError} if a provider-configuration file

* violates the specified format, or if it names a provider class that

* cannot be found and instantiated, or if the result of instantiating the

* class is not assignable to the service type, or if any other kind of

* exception or error is thrown as the next provider is located and

* instantiated. To write robust code it is only necessary to catch {@link

* ServiceConfigurationError} when using a service iterator.

*

*

If such an error is thrown then subsequent invocations of the

* iterator will make a best effort to locate and instantiate the next

* available provider, but in general such recovery cannot be guaranteed.

*

*

* style="padding-right: 1em; font-weight: bold">Design Note

* Throwing an error in these cases may seem extreme. The rationale for

* this behavior is that a malformed provider-configuration file, like a

* malformed class file, indicates a serious problem with the way the Java

* virtual machine is configured or is being used. As such it is

* preferable to throw an error rather than try to recover or, even worse,

* fail silently.

*

*

The iterator returned by this method does not support removal.

* Invoking its {@link java.util.Iterator#remove() remove} method will

* cause an {@link UnsupportedOperationException} to be thrown.

*

* @implNote When adding providers to the cache, the {@link #iterator

* Iterator} processes resources in the order that the {@link

* java.lang.ClassLoader#getResources(java.lang.String)

* ClassLoader.getResources(String)} method finds the service configuration

* files.

*

* @return An iterator that lazily loads providers for this loader's

* service

*/

public Iterator iterator() {

return new Iterator() {

Iterator> 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();

}

};

}

我们用到的迭代器其实是一个Map：

// Cached providers, in instantiation order

private LinkedHashMap providers = new LinkedHashMap<>();

它用来缓存加载的实现类，真正执行的是lookupIterator：

// The current lazy-lookup iterator

private LazyIterator lookupIterator;

我们看下它的hasNext和next方法：

public boolean hasNext() {

if (acc == null) {

return hasNextService();

} else {

PrivilegedAction action = new PrivilegedAction() {

public Boolean run() { return hasNextService(); }

};

return AccessController.doPrivileged(action, acc);

}

}

public S next() {

if (acc == null) {

return nextService();

} else {

PrivilegedAction action = new PrivilegedAction() {

public S run() { return nextService(); }

};

return AccessController.doPrivileged(action, acc);

}

}

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(); // This cannot happen

}

public boolean hasNext() {

if (acc == null) {

return hasNextService();

} else {

PrivilegedAction action = new PrivilegedAction() {

public Boolean run() { return hasNextService(); }

};

return AccessController.doPrivileged(action, acc);

}

}

hasNext查找实现类，并指定了类路径：

private static final String PREFIX = "META-INF/services/";

具体查找操作看这里：

pending = parse(service, configs.nextElement());

next则是实例化加载到的实现类，使用反射Class.forName加载类、newInstance实例化对象。

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