JDK有個ServiceLoader類，在java.util包裡，支援按約定目錄/META-INF/services去找到介面全路徑命名的檔案，讀取檔案內容得到介面實現類的全路徑，載入並例項化。如果我們在自己的程式碼中定義一個介面，別人按介面實現並打包好了，那麼我們只需要引入jar包，通過ServiceLoader就能夠把別人的實現用起來。舉個例子，JDK中的JDBC提供一個數據庫連線驅動介面，不同的廠商可以有不同的實現，如果它們給的jar包裡按規定提供了配置和實現類，那麼我們就可以執行不同的資料庫連線操作，比如MySql的jar包裡就會有自己的配置：

　　這裡檔名就是介面：

　　 檔案內容是實現類：

　　我們自己實現一個簡單例子，不需要打jar包，把目錄放到spring boot的resources下即可，這裡就是classpath，跟你放jar包裡效果一樣。

　　1、定義一個介面：

package com.wlf.service;

public interface ITest {
    void saySomething();
}

　　2、定義兩個實現：

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、按預定新增/META-INF/services/com.wlf.service.ITest檔案：

com.wlf.service.impl.ITestImpl1
com.wlf.service.impl.ITestImpl2

　　4、定義一個執行類，通過ServiceLoader載入並例項化，呼叫實現類方法，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<ITest> serviceLoader = ServiceLoader.load(ITest.class);
        Iterator<ITest> iTests = serviceLoader.iterator();
        while (iTests.hasNext()) {
            ITest iTest = iTests.next();
            System.out.printf("loading %s\n", iTest.getClass().getName());
            iTest.saySomething();
        }
    }
}

　　列印結果：

　　ServiceLoader原始碼比較簡單，可以看下上面我們使用到的標黃了的方法：

    /**
     * Lazily loads the available providers of this loader's service.
     *
     * <p> 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.
     *
     * <p> 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.
     *
     * <p> 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.
     *
     * <blockquote style="font-size: smaller; line-height: 1.2"><span
     * style="padding-right: 1em; font-weight: bold">Design Note</span>
     * 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.</blockquote>
     *
     * <p> 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<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();
            }

        };
    }

　　我們用到的迭代器其實是一個Map：

    // Cached providers, in instantiation order
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

　　它用來快取載入的實現類，真正執行的是lookupIterator：

    // The current lazy-lookup iterator
    private LazyIterator lookupIterator;

　　我們看下它的hasNext和next方法：

        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);
            }
        }

        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<Boolean> action = new PrivilegedAction<Boolean>() {
                    public Boolean run() { return hasNextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }

　　hasNext查詢實現類，並指定了類路徑：

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

　　具體查詢操作看這裡：

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

　　next則是例項化載入到的實現類，使用反射Class.forName載入類、newInstance例項化物件。