Esper入门简介：一

先确定理解了Esper的思想下：

**

* Chapter 1. Technology Overview

*

***** 1.关系数据库及其sql设计的目地主要是为了数据相对静态和复杂查询比较少的应用（OLTP，磁盘数据的存取有相应的优化结构，当然内存数据库，数据存放在内存内）。

* 对于cep系统来说，内存数据库更接近其目地。

*

***** 2.cep引擎的实现和关系数据库的实现有点相反，数据库一般把数据存放到磁盘（静态的），我们通过网络把sql流传给数据库引擎，操作数据库，返回结果流，流向应用。

* 而cep则是把查询表达式存储，数据事件流流向其中。其实查询表达式中的窗口函数及表达式等组合起来，实现了一种条件性数据结构（时间范围或符合条件的数据才能流向其中存储）。

*

***** 3.cep提供了两种机制处理事件：事件模式和事件流查询。

根据官方网站给的入门例子：

主代码：

package com.doctor.esper.tutorial;

import com.espertech.esper.client.Configuration;
import com.espertech.esper.client.EPServiceProvider;
import com.espertech.esper.client.EPServiceProviderManager;
import com.espertech.esper.client.EPStatement;

/**
 * code for
 *
 * @see http://www.espertech.com/esper/quickstart.php
 *
 * @author doctor
 *
 * @time 2015年5月28日 下午3:51:18
 */
public class QuickStart {

	public static void main(String[] args) {

		// Configuration
		//
		// Esper runs out of the box and no configuration is required. However configuration can help make statements more readable and provides the
		// opportunity to plug-in extensions and to configure relational database access.
		//
		// One useful configuration item specifies Java package names from which to take event classes.
		//
		// This snippet of using the configuration API makes the Java package of the OrderEvent class known to an engine instance:
		// In order to query the OrderEvent events, we can now remove the package name from the statement:see line40

		Configuration configuration = new Configuration();
		configuration.addEventTypeAutoName("com.doctor.esper.tutorial");

		// Creating a Statement
		// A statement is a continuous query registered with an Esper engine instance that provides results to listeners as new data arrives, in
		// real-time, or by demand via the iterator (pull) API.
		// The next code snippet obtains an engine instance and registers a continuous query. The query returns the average price over all OrderEvent
		// events that arrived in the last 30 seconds:
		EPServiceProvider epServiceProvider = EPServiceProviderManager.getDefaultProvider(configuration);
		String expression = "select avg(price) from OrderEvent.win:time(30 sec)";
		EPStatement epStatement = epServiceProvider.getEPAdministrator().createEPL(expression);

		// By attaching the listener to the statement the engine provides the statement's results to the listener:
		MyListener myListener = new MyListener();
		epStatement.addListener(myListener);

		// Sending events
		// The runtime API accepts events for processing. As a statement's results change, the engine indicates the new results to listeners right
		// when the events are processed by the engine.
		// Sending events is straightforward as well:
		OrderEvent orderEvent = new OrderEvent("shirt", 75.50D);
		epServiceProvider.getEPRuntime().sendEvent(orderEvent);

	}

}

定义事件（java普通类形式）：

package com.doctor.esper.tutorial;

import com.alibaba.fastjson.JSON;

/**
 * Creating a Java Event Class
 *
 * Java classes are a good choice for representing events, however Map-based or XML event representations can also be good choices depending on
 * your
 * architectural requirements.
 *
 * A sample Java class that represents an order event is shown below. A simple plain-old Java class that provides getter-methods for access to
 * event
 * properties works best:
 *
 * @author doctor
 *
 * @time 2015年5月28日 下午3:59:02
 */
public class OrderEvent {
	private String itemName;
	private double price;

	public OrderEvent(String itemName, double price) {
		this.itemName = itemName;
		this.price = price;
	}

	public String getItemName() {
		return itemName;
	}

	public double getPrice() {
		return price;
	}

	@Override
	public String toString() {
		return JSON.toJSONString(this);
	}
}

监听器/触发器：

/**
 * Adding a Listener
 *
 * Listeners are invoked by the engine in response to one or more events that change a statement's result set. Listeners implement the UpdateListener
 * interface and act on EventBean instances as the next code snippet outlines
 *
 * @author doctor
 *
 * @time 2015年5月28日 下午4:02:37
 */
public class MyListener implements UpdateListener {

	@Override
	public void update(EventBean[] newEvents, EventBean[] oldEvents) {
		EventBean eventBean = newEvents[0];
		System.out.println("avg = " + eventBean.get("avg(price)"));

	}

}

运行下结果：

06-20 16:03:27.702 main  INFO  c.e.e.c.s.EPServiceProviderImpl -    Initializing engine URI ‘default‘ version 5.2.0

avg = 75.5