「DUBBO系列」JDK SPI机制原理

系统管理员 2021-09-24 23:36 264阅读 0赞

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# 1 文章概述 #

SPI（Service Provider Interface）是一种服务发现机制，本质是将接口实现类的全限定名配置在文件中，并由服务加载器读取配置文件加载实现类，这样可以在运行时动态为接口替换实现类，我们通过 SPI 机制可以为程序提供拓展功能。本文我们介绍JDK SPI使用方法并通过分析源码深入理解。后续文章介绍Dubbo自己实现的SPI机制。

# 2 SPI实例 #

(1) 新建项目工程并定义接口DataBaseDriver

public interface DataBaseDriver { 
        String connect(String hostIp);
    }

(2) 打包这个工程为JAR

<dependency>
      <groupId>com.itxpz.spi</groupId>
      <artifactId>DataBaseDriver</artifactId>
      <version>1.0.0-SNAPSHOT</version>
    </dependency>

(3) 新建MySQLDriver工程添加上述依赖并实现DataBaseDriver接口

import com.itxpz.database.driver.DataBaseDriver;
    public class MySQLDataBaseDriver implements DataBaseDriver { 
        @Override
        public String connect(String hostIp) { 
            return "MySQL DataBase Driver connect";
        }
    }

(4) 在MySQLDriver项目新建文件

src/main/resources/META-INF/services/com.itxpz.database.driver.DataBaseDriver

(5) 在此文件添加如下内容

com.itxpz.database.mysql.driver.MySQLDataBaseDriver

(6) 新建OracleDriver工程操作方式相同，配置文件内容有所变化

com.itxpz.database.oracle.driver.OracleDataBaseDriver

(7) 将上述两个项目打包

<dependency>
      <groupId>com.itxpz.spi</groupId>
      <artifactId>MySQLDriver</artifactId>
      <version>1.0.0-SNAPSHOT</version>
    </dependency>
    <dependency>
      <groupId>com.itxpz.spi</groupId>
      <artifactId>OracleDriver</artifactId>
      <version>1.0.0-SNAPSHOT</version>
    </dependency>

(8) 新建测试项目引入上述依赖并执行以下代码

public class DataBaseConnector { 
        public static void main(String[] args) { 
            ServiceLoader<DataBaseDriver> serviceLoader = ServiceLoader.load(DataBaseDriver.class);
            Iterator<DataBaseDriver> iterator = serviceLoader.iterator();
            while (iterator.hasNext()) { 
                DataBaseDriver driver = iterator.next();
                System.out.println(driver.connect("localhost"));
            }
        }
    }
    
    输出结果
    MySQL DataBase Driver connect
    Oracle DataBase Driver connect

我们并没有指定使用哪个驱动进行连接，而是通过ServiceLoader方式加载实现了DataBaseDriver接口的实现类。假设我们只想要使用MySQL驱动那么直接引入相应依赖即可。

# 3 源码分析 #

## 3.1 迭代器模式 ##

我们在分析JDK SPI源码之前首先学习迭代器设计模式，因为JDK SPI应用了迭代器模式。

public class OrderInfoModel implements Serializable { 
        private String orderId;
    
        public OrderInfoModel(String orderId) { 
            this.orderId = orderId;
        }
    
        public String getOrderId() { 
            return orderId;
        }
    
        public void setOrderId(String orderId) { 
            this.orderId = orderId;
        }
    
        @Override
        public String toString() { 
            return "OrderInfoModel [orderId=" + orderId + "]";
        }
    }
    
    public class OrderInfoIterator implements Iterator<OrderInfoModel> { 
        private int cursor;
        private List<OrderInfoModel> orderInfoList;
    
        public OrderInfoIterator(List<OrderInfoModel> orderInfoList) { 
            this.cursor = 0;
            this.orderInfoList = orderInfoList;
        }
    
        @Override
        public boolean hasNext() { 
            if(CollectionUtils.isEmpty(orderInfoList)) { 
                throw new RuntimeException("param error");
            }
            return cursor != orderInfoList.size();
        }
    
        @Override
        public OrderInfoModel next() { 
            if(CollectionUtils.isEmpty(orderInfoList)) { 
                throw new RuntimeException("param error");
            }
            OrderInfoModel element = orderInfoList.get(cursor);
            cursor++;
            return element;
        }
    }
    
    public class TestMain { 
        public static void main(String[] args) { 
            List<OrderInfoModel> orderInfoList = new ArrayList<>();
            OrderInfoModel order1 = new OrderInfoModel("111");
            OrderInfoModel order2 = new OrderInfoModel("222");
            OrderInfoModel order3 = new OrderInfoModel("333");
            orderInfoList.add(order1);
            orderInfoList.add(order2);
            orderInfoList.add(order3);
    
            Iterator<OrderInfoModel> iterator = new OrderInfoIterator(orderInfoList);
            while (iterator.hasNext()) { 
                System.out.println(iterator.next());
            }
        }
    }
    
    输出结果
    OrderInfoModel [orderId=111]
    OrderInfoModel [orderId=222]
    OrderInfoModel [orderId=333]

## 3.2 SPI源码分析 ##

public class DataBaseConnector { 
        public static void main(String[] args) { 
            // 根据类型获取服务加载器
            ServiceLoader<DataBaseDriver> serviceLoader = ServiceLoader.load(DataBaseDriver.class);
            // 获取迭代器
            Iterator<DataBaseDriver> iterator = serviceLoader.iterator();
            // 迭代器遍历
            while (iterator.hasNext()) { 
                DataBaseDriver driver = iterator.next();
                System.out.println(driver.connect("localhost"));
            }
        }
    }

进入ServiceLoader.load方法

ServiceLoader<DataBaseDriver> serviceLoader = ServiceLoader.load(DataBaseDriver.class);

跟进load方法发现只是进行初始化

public final class ServiceLoader<S> implements Iterable<S> { 
    
        // 默认加载服务路径
        private static final String PREFIX = "META-INF/services/";
    
        // 缓存提供者信息
        private LinkedHashMap<String, S> providers = new LinkedHashMap<>();
    
        // 当前迭代器
        private LazyIterator lookupIterator;
    
        public void reload() { 
            // 清除缓存
            providers.clear();
            // 核心迭代器
            lookupIterator = new LazyIterator(service, loader);
        }
    
        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();
        }
    }

进入serviceLoader.iterator()方法

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() { 
    			// 如果缓存有则取缓存
    			if (knownProviders.hasNext())
    				return knownProviders.next().getValue();
    			// 缓存没有则重新加载
    			return lookupIterator.next();
    		}
    
    		public void remove() { 
    			throw new UnsupportedOperationException();
    		}
    	}
    }

进入迭代器遍历代码

while (iterator.hasNext()) { 
        DataBaseDriver driver = iterator.next();
        System.out.println(driver.connect("localhost"));
    }

LazyIterator核心方法分析详见注释。核心是读取指定路径文件内容，通过反射进行类实例化并且保存至缓存容器。因为创建类需要使用栈空间，如果不使用缓存频繁创建类会造成栈溢出异常。

private class LazyIterator implements Iterator<S> { 
        Class<S> service;
        ClassLoader loader;
        Enumeration<URL> configs = null;
        Iterator<String> pending = null;
        String nextName = null;
    
        private boolean hasNextService() { 
            if (nextName != null) { 
                return true;
            }
            if (configs == null) { 
                try { 
                    // META-INFO/Services/com.itxpz.database.driver.DataBaseDriver
                    String fullName = PREFIX + service.getName();
                    if (loader == null)
                        configs = ClassLoader.getSystemResources(fullName);
                    else
                        // 构建fullName路径配置对象
                        configs = loader.getResources(fullName);
                } catch (IOException x) { 
                    fail(service, "Error locating configuration files", x);
                }
            }
            while ((pending == null) || !pending.hasNext()) { 
                if (!configs.hasMoreElements()) { 
                    return false;
                }
                // 解析文件内容
                pending = parse(service, configs.nextElement());
            }
            // com.itxpz.database.mysql.driver.MySQLDataBaseDriver
            // com.itxpz.database.mysql.driver.OracleDataBaseDriver
            nextName = pending.next();
            return true;
        }
    
        private S nextService() { 
            if (!hasNextService())
                throw new NoSuchElementException();
            // com.itxpz.database.mysql.driver.MySQLDataBaseDriver
            // com.itxpz.database.mysql.driver.OracleDataBaseDriver
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try { 
                // 通过反射进行实例化
                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 { 
                // 类型转换父类引用指向子类对象
                S p = service.cast(c.newInstance());
                // 保存至缓存容器
                providers.put(cn, p);
                return p;
            } catch (Throwable x) { 
                fail(service, "Provider " + cn + " could not be instantiated", x);
            }
            throw new Error();
        }
    }

# 4 实际应用 #

使用JDBC时利用DriverManager加载数据库驱动时正是使用了SPI机制，我们引入MySQL依赖

<dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>6.0.6</version>
    </dependency>

在MySQL依赖包中会发现如下文件

META-INF/services/java.sql.Driver

DriverManager加载驱动时可以发现SPI机制

package java.sql;
    public class DriverManager { 
        private static void loadInitialDrivers() { 
            AccessController.doPrivileged(new PrivilegedAction<Void>() { 
                public Void run() { 
                    // META-INF/services/java.sql.Driver
                    ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                    Iterator<Driver> driversIterator = loadedDrivers.iterator();
                    try { 
                        while(driversIterator.hasNext()) { 
                            driversIterator.next();
                        }
                    } catch(Throwable t) { 
                    }
                    return null;
                }
            });
        }
    }