Java SPI原理与源码分析

public interface IHello {
    /**
     *  sayHello
     */
    void sayHello();

}

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public class ChineseHello implements IHello {
    @Override
    public void sayHello() {
        System.out.println("你好");

    }
}

public class EnglishHello implements IHello {
    @Override
    public void sayHello() {
        System.out.println("hello");
    }
}

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xxx.spi.ChineseHello
xxx.spi.EnglishHello

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    public static void main(String[] args) {
        ServiceLoader serviceLoader = ServiceLoader.load(IHello.class);
        Iterator iterator = serviceLoader.iterator();
        while (iterator.hasNext()) {
            IHello hello = iterator.next();
            hello.sayHello();
        }
    }

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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();
        }
    };
}

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while(hasNext()) {
    next();
}

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public boolean hasNext() {
    if (acc == null) {
        return hasNextService();
    } else {
        PrivilegedAction action = new PrivilegedAction() {
            public Boolean run() { return hasNextService(); }
        };
        return AccessController.doPrivileged(action, acc);
    }
}

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private boolean hasNextService() {
    //默认为空,当调用完hasNext再调用next方法就会有值,直接返回
    if (nextName != null) {
        return true;
    }
    if (configs == null) {
        try {
            //META-INF/services/xxx.interface
            String fullName = PREFIX + service.getName();
            // 加载文件
            if (loader == null)
                configs = ClassLoader.getSystemResources(fullName);
            else
                configs = loader.getResources(fullName);
        } catch (IOException x) {
            fail(service, "Error locating configuration files", x);
        }
    }
    //遍历并解析文件内容
    while ((pending == null) || !pending.hasNext()) {
        if (!configs.hasMoreElements()) {
            return false;
        }
        pending = parse(service, configs.nextElement());
    }
    nextName = pending.next();
    return true;
}

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private Iterator parse(Class> service, URL u)
    throws ServiceConfigurationError
{
    InputStream in = null;
    BufferedReader r = null;
    ArrayList 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();
}

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private int parseLine(Class> service, URL u, BufferedReader r, int lc,
                      List 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);
        }
        if (!providers.containsKey(ln) && !names.contains(ln))
            names.add(ln);
    }
    return lc + 1;
}

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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
}

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/**
 * Load the initial JDBC drivers by checking the System property
 * jdbc.properties and then use the {@code ServiceLoader} mechanism
 */
static {
    loadInitialDrivers();
    println("JDBC DriverManager initialized");
}

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private static void loadInitialDrivers() {
    String drivers;
    try {
        drivers = AccessController.doPrivileged(new PrivilegedAction() {
            public String run() {
                return System.getProperty("jdbc.drivers");
            }
        });
    } catch (Exception ex) {
        drivers = null;
    }
    // If the driver is packaged as a Service Provider, load it.
    // Get all the drivers through the classloader
    // exposed as a java.sql.Driver.class service.
    // ServiceLoader.load() replaces the sun.misc.Providers()

    AccessController.doPrivileged(new PrivilegedAction() {
        public Void run() {
      //通过ServiceLoader加载驱动类
            ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);
            Iterator driversIterator = loadedDrivers.iterator();

            /* Load these drivers, so that they can be instantiated.
             * It may be the case that the driver class may not be there
             * i.e. there may be a packaged driver with the service class
             * as implementation of java.sql.Driver but the actual class
             * may be missing. In that case a java.util.ServiceConfigurationError
             * will be thrown at runtime by the VM trying to locate
             * and load the service.
             *
             * Adding a try catch block to catch those runtime errors
             * if driver not available in classpath but it's
             * packaged as service and that service is there in classpath.
             */
            try{
                while(driversIterator.hasNext()) {
                    driversIterator.next();
                }
            } catch(Throwable t) {
            // Do nothing
            }
            return null;
        }
    });
    println("DriverManager.initialize: jdbc.drivers = " + drivers);
    if (drivers == null || drivers.equals("")) {
        return;
    }
    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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