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Skywalking-12: skywalking SPI mechanism

2021-10-14 05:38:44 Switch

SPI Mechanism

Basic overview

SPI Full name Service Provider Interface , Is a service discovery mechanism . By providing interfaces 、 Predefined loaders ( Loader ) And the commonly known configuration ( Generally in META-INF Under the table of contents ), You can dynamically load service implementation classes .

Class diagram

file

Through the class diagram, we can analyze , ServiceLoader Realized Iterable Interface , Provides the function of iteration .

and ServiceLoader Delegate the implementation of the iteration to LazyIterator .

LazyIterator Provides the ability to delay iterations , When there is a need , To load .

stay Skywalking Use in modules

Interface definition

org.apache.skywalking.oap.server.library.module.ModuleDefine

package org.apache.skywalking.oap.server.library.module;

import java.lang.reflect.Field;
import java.util.Enumeration;
import java.util.Properties;
import java.util.ServiceLoader;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * A module definition.
 */
public abstract class ModuleDefine implements ModuleProviderHolder {

    private static final Logger LOGGER = LoggerFactory.getLogger(ModuleDefine.class);

    private ModuleProvider loadedProvider = null;

    private final String name;

    public ModuleDefine(String name) {
        this.name = name;
    }

    /**
     * @return the module name
     *
     */
    public final String name() {
        return name;
    }

    /**
     * @return the {@link Service} provided by this module.
     */
    public abstract Class[] services();

    /**
     * Run the prepare stage for the module, including finding all potential providers, and asking them to prepare.
     *
     * @param moduleManager of this module
     * @param configuration of this module
     * @throws ProviderNotFoundException when even don't find a single one providers.
     */
    void prepare(ModuleManager moduleManager, ApplicationConfiguration.ModuleConfiguration configuration,
        ServiceLoader<ModuleProvider> moduleProviderLoader) throws ProviderNotFoundException, ServiceNotProvidedException, ModuleConfigException, ModuleStartException {
        // etc...
    }
    
    // etc...

    @Override
    public final ModuleProvider provider() throws DuplicateProviderException, ProviderNotFoundException {
        if (loadedProvider == null) {
            throw new ProviderNotFoundException("There is no module provider in " + this.name() + " module!");
        }
        return loadedProvider;
    }
}

Interface implementation

org.apache.skywalking.oap.server.library.module.BaseModuleA

package org.apache.skywalking.oap.server.library.module;

public class BaseModuleA extends ModuleDefine {

    public BaseModuleA() {
        super("BaseA");
    }

    //  Need to provide service interface 
    @Override
    public Class<? extends Service>[] services() {
        return new Class[] {
            ServiceABusiness1.class,
            ServiceABusiness2.class
        };
    }

    public interface ServiceABusiness1 extends Service {
        void print();
    }

    public interface ServiceABusiness2 extends Service {
    }
}

META-INF Definition

META-INF/services/org.apache.skywalking.oap.server.library.module.ModuleDefine

org.apache.skywalking.oap.server.library.module.BaseModuleA

Usage mode

org.apache.skywalking.oap.server.library.module.ModuleManager#init

    /**
     * Init the given modules
     */
    public void init(ApplicationConfiguration applicationConfiguration) /* etc... */ {
        // SPI Mechanism loading 
        ServiceLoaderModuleDefine> moduleServiceLoader = ServiceLoader.load(ModuleDefine.class);
        //  Iterator gets 
        for (ModuleDefine module : moduleServiceLoader) {
            // do something
            // etc...
        }
        // etc...
    }

The source code parsing

package java.util;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.PrivilegedAction;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

public final class ServiceLoader<S> implements Iterable<S> {
    //  Directory prefix 
    private static final String PREFIX = "META-INF/services/";

    //  Object to be loaded Class object 
    private final Class<S> service;

    //  Class loader 
    private final ClassLoader loader;

    // The access control context taken when the ServiceLoader is created
    private final AccessControlContext acc;

    //  Load object cache ( Sort by instantiation order )
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

    //  Lazy load iterator currently in use 
    private LazyIterator lookupIterator;

    //  heavy load 
    public void reload() {
        //  Clear the load object cache 
        providers.clear();
        //  Reset lazy load iterator 
        lookupIterator = new LazyIterator(service, loader);
    }

    //  Direct creation of ServiceLoader object , Only through loadXXX obtain ServiceLoader object 
    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();
    }

    private static void fail(Class<?> service, String msg, Throwable cause) throws ServiceConfigurationError {
        throw new ServiceConfigurationError(service.getName() + ": " + msg, cause);
    }

    private static void fail(Class<?> service, String msg) throws ServiceConfigurationError {
        throw new ServiceConfigurationError(service.getName() + ": " + msg);
    }

    private static void fail(Class<?> service, URL u, int line, String msg) throws ServiceConfigurationError {
        fail(service, u + ":" + line + ": " + msg);
    }

    //  Parse a line in the configuration file , If there is no comment , Then it is added to the class name list 
    private int parseLine(Class<?> service, URL u, BufferedReader r, int lc, List<String> 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;
    }

    //  Parse configuration file , Returns a list of implementation class names 
    private Iterator<String> parse(Class<?> service, URL u) throws ServiceConfigurationError {
        InputStream in = null;
        BufferedReader r = null;
        ArrayList<String> 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();
    }

    //  Lazy load iterators , Provides the ability to delay iterations , When there is a need , To load 
    private class LazyIterator implements Iterator<S> {
        //  Object to be loaded Class object 
        Class<S> service;
        //  Class loader 
        ClassLoader loader;
        //  Profile list 
        Enumeration<URL> configs = null;
        //  Iterator for the list of class names in the configuration file of the current iteration 
        Iterator<String> pending = null;
        //  The name of the next implementation class 
        String nextName = null;

        private LazyIterator(Class<S> service, ClassLoader loader) {
            this.service = service;
            this.loader = loader;
        }

        //  Is there a next Service
        private boolean hasNextService() {
            if (nextName != null) {
                return true;
            }
            //  Load all configuration files 
            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);
                }
            }
            //  When the current class name list is iterated , Load the next configuration file 
            while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                pending = parse(service, configs.nextElement());
            }
            //  Get the next class name 
            nextName = pending.next();
            return true;
        }

        //  Get the next one Service
        private S nextService() {
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                //  Class name  ->  Class Class object 
                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 {
                //  Instantiation 
                S p = service.cast(c.newInstance());
                //  Add to cache 
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                fail(service, "Provider " + cn + " could not be instantiated", x);
            }
            throw new Error();          // This cannot happen
        }

        //  iterator , Whether there is the next element 
        public boolean hasNext() {
            if (acc == null) {
                return hasNextService();
            } else {
                //  Authorized resources 
                PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {
                    public Boolean run() { return hasNextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }

        //  iterator , Get the next element 
        public S next() {
            if (acc == null) {
                return nextService();
            } else {
                //  Authorized resources 
                PrivilegedAction<S> action = new PrivilegedAction<S>() {
                    public S run() { return nextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }

        //  Deleting... Is not supported 
        public void remove() {
            throw new UnsupportedOperationException();
        }

    }

    //  Iterator implementation , If there is a cache, get it from the cache , If not, load... From the lazy load iterator 
    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();
            }
        };
    }

    //  By class Class Object and class loading , obtain ServiceLoader
    public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader) {
        return new ServiceLoader<>(service, loader);
    }

    //  By class Class object , obtain ServiceLoader
    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

    //  By class Class Object and extension class loaders , obtain ServiceLoader
    public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
        ClassLoader cl = ClassLoader.getSystemClassLoader();
        ClassLoader prev = null;
        while (cl != null) {
            prev = cl;
            cl = cl.getParent();
        }
        return ServiceLoader.load(service, prev);
    }

    public String toString() {
        return "java.util.ServiceLoader[" + service.getName() + "]";
    }

}

PS: JDK  Provided SPI  Mechanism , You must use iterators to traverse to get the required implementation , and Dubbo SPI Can pass #getExtension  Gets the specified implementation class .

summary

Through source code analysis , You can see that Skywalking I didn't define my own SPI Mechanism , But read deeply Skywalking After the use scenario , Discovery use JDK Provided SPI There's nothing wrong with the mechanism .

Personally think that , Any technology should be selected according to the scene , What suits is the best , If there were no such complex needs , There's no need to look like dubbo  equally , Define your own SPI  Mechanism .

Reference documents

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https://chowdera.com/2021/10/20211002145413001m.html

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