[Java知识库]Java SPI实现

JDBC使用SPI

/**
 * @author zhangcheng
 * @date 2021/8/12
 */
public class T01SPI {
    public static void main(String[] args) {
        Enumeration<Driver> drivers = DriverManager.getDrivers();
        Driver driver;
        while (drivers.hasMoreElements()) {
            driver = drivers.nextElement();
            // sun.misc.Launcher$AppClassLoader@18b4aac2
            System.out.println(driver.getClass() + "==>类加载器: " + driver.getClass().getClassLoader());
        }
        // BootstrapClassLoader
        System.out.println("DriverManager的类加载器:" + DriverManager.class.getClassLoader());
    }
}

输出结果如下:

class com.mysql.cj.jdbc.Driver==>类加载器: sun.misc.Launcher$AppClassLoader@18b4aac2
class com.alibaba.druid.proxy.DruidDriver==>类加载器: sun.misc.Launcher$AppClassLoader@18b4aac2
class com.alibaba.druid.mock.MockDriver==>类加载器: sun.misc.Launcher$AppClassLoader@18b4aac2
class org.h2.Driver==>类加载器: sun.misc.Launcher$AppClassLoader@18b4aac2
DriverManager的类加载器:null

DriverManager使用的类加载器为null，即通过Bootstrap ClassLoader加载进来的;
而com.mysql.jdbc.Driver是通过Application ClassLoader加载进来的。

由于双亲委派模型，父加载器是拿不到通过子加载器加载的类的。

首先用户自定义的SPI拓展肯定不能通过Bootstrap ClassLoader加载,因为它又不是系统类.
正常通过Application ClassLoader加载.

这里就引出了一个问题，为什么通过Bootstrap ClassLoader加载进来的DriverManager，可以拿到Application ClassLoader加载进来的com.mysql.jdbc.Driver？

这个现象，就是破坏了双亲委派模型。

为什么要破坏双亲委派模型

因为DriverManager是通过Bootstrap ClassLoader加载进来的，
而com.mysql.jdbc.Driver是通过classpath的JAR包加载进来的。
要想通过DriverManager获得 Driver 的具体实现，必须破坏双亲委派模型才能拿到。

DriverManager是如何拿到com.mysql.jdbc.Driver类的

通过Thread.currentThread().setContextClassLoader()，
将Application ClassLoader设置为线程上下文加载器。
在DriverManager类里通过Thread.currentThread().getContextClassLoader()
拿到Application ClassLoader进行加载。

DriverManager静态代码块

static {
    loadInitialDrivers();
    println("JDBC DriverManager initialized");
}

loadInitialDrivers()里

ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
Iterator<Driver> driversIterator = loadedDrivers.iterator();

java.util.ServiceLoader

/**
    * Creates a new service loader for the given service type, using the
    * current thread's {@linkplain java.lang.Thread#getContextClassLoader
    * context class loader}.
    *
    * <p> An invocation of this convenience method of the form
    *
    * <blockquote><pre>
    * ServiceLoader.load(<i>service</i>)</pre></blockquote>
    *
    * is equivalent to
    *
    * <blockquote><pre>
    * ServiceLoader.load(<i>service</i>,
    *                    Thread.currentThread().getContextClassLoader())</pre></blockquote>
    *
    * @param  <S> the class of the service type
    *
    * @param  service
    *         The interface or abstract class representing the service
    *
    * @return A new service loader
    */
public static <S> ServiceLoader<S> load(Class<S> service) {
    // 线程上下文类加载器是 Launcher$AppClassLoader
    ClassLoader cl = Thread.currentThread().getContextClassLoader();
    // service: java.sql.Driver
    return ServiceLoader.load(service, cl);
}

load:537, ServiceLoader (java.util)
run:586, DriverManager$2 (java.sql)
run:583, DriverManager$2 (java.sql)
doPrivileged:-1, AccessController (java.security)
loadInitialDrivers:583, DriverManager (java.sql)
<clinit>:101, DriverManager (java.sql)
main:13, T01SPI (com.zc.basic.z18spi)

入口: ServiceLoader.load()

默认线程创建时候的上下文类加载器就是 Launcher$AppClassLoader

ServiceLoader.load() 方法里全程用的是 传入的 上下文类加载器.

/**为给定的服务类型创建一个新的服务加载程序，使用
*当前线程的{@linkplain java.lang.Thread#getContextClassLoader
*上下文类装入器}。
*/
public static <S> ServiceLoader<S> load(Class<S> service) {
    //1.获取当前线程的类加载
    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);
}

// 构造函数
private ServiceLoader(Class<S> svc, ClassLoader cl) {
    // 判断入参是否为null
    service = Objects.requireNonNull(svc, "Service interface cannot be null");
    // 2.加载器如果不存在，获取系统类加载器
    loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
    // 3.获取访问控制器
    acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
    reload();
}

public void reload() {
    // 清空缓存
    providers.clear();
    // 初始化内部类，用于遍历提供者
    lookupIterator = new LazyIterator(service, loader);
}

属性

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

// 要加载的类
private final Class<S> service;

// 用于加载实现类的类加载器
private final ClassLoader loader;

// 访问控制器
private final AccessControlContext acc;

// 提供者的缓存
private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

// 一个内部类，用于遍历实现类
private LazyIterator lookupIterator;

现在我们发现重点就在于LazyIterator这个内部类上.

private class LazyIterator
    implements Iterator<S> {

    Class<S> service;
    ClassLoader loader;
    Enumeration<URL> configs = null;
    Iterator<String> pending = null;
    String nextName = null;

    private LazyIterator(Class<S> service, ClassLoader loader) {
        this.service = service;
        this.loader = loader;
    }
    
    private boolean hasNextService() {
        if (nextName != null) {
            return true;
        }
        if (configs == null) {
            try {
                // 获取META-INF/services下文件全称
                String fullName = PREFIX + service.getName();
                if (loader == null)
                    configs = ClassLoader.getSystemResources(fullName);
                else
                    // 获取配置文件内具体实现的枚举类
                    configs = loader.getResources(fullName);
            } catch (IOException x) {
                fail(service, "Error locating configuration files", x);
            }
        }
        while ((pending == null) || !pending.hasNext()) {
            if (!configs.hasMoreElements()) {
                return false;
            }
            // 解析配置
            pending = parse(service, configs.nextElement());
        }
        nextName = pending.next();
        return true;
    }

    private Iterator<String> parse(Class<?> service, URL u)
        throws ServiceConfigurationError {
        InputStream in = null;
        BufferedReader r = null;
        // 存储配置文件中实现类的全限定名
        ArrayList<String> names = new ArrayList<>();
        try {
            in = u.openStream();
            r = new BufferedReader(new InputStreamReader(in, "utf-8"));
            int lc = 1;
            // 读取文件内容
            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();
    }

private S nextService() {
    if (!hasNextService())
        throw new NoSuchElementException();
    // 循环遍历获取实现类的全限定名
    String cn = nextName;
    nextName = null;
    Class<?> c = null;
    try {
        // 实例化实现类
        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 {
        // 将实例化的类强转成所表示的类型
        S p = service.cast(c.newInstance());
        // 缓存实现类
        providers.put(cn, p);
        // 返回对象
        return p;
    } catch (Throwable x) {
        fail(service,
             "Provider " + cn + " could not be instantiated",
             x);
    }
    throw new Error();          // This cannot happen
}