摘要： 如何像写一个普通程序一样写Java服务端程序

全民学后端快餐教程(1) - 只不过是写个Java程序

服务端程序本质上也只是个Java程序，它接收客户端的输入，然后将计算处理后的返回值返回给客户端。下面我们就以这个思路开始Java后端之旅吧。

引用Spring Boot库

处理HTTP请求之类的事情，我们需要库的帮助。所以第一步我们就把Spring Boot引入进来。

不需要任何工具，我们使用maven来管理库依赖，这样我们只要写一个pom.xml就好了。我们先写一个最简的pom.xml。主要是定义groupId，比如是我司，还有artifactId，就是应用的具体名字：

<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>cn.alios.system.service.prefix</groupId> <artifactId>Prefix</artifactId> <version>1.0.0-SNAPSHOT</version></project>

添加父引用

类似于类的继承，我们不是从头开发，而是继承Spring Boot Starter框架。添加parent的内容如下：

 <parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>2.1.2.RELEASE</version> </parent>

2.1.2是本文写作时，Spring Boot的最新版本。

增加依赖

为了自动下载库，我们将需要的几个库添加到pom.xml中的依赖项中。这样maven就可以帮我们从仓库中下载最新的库代码。

我们需要AOP和Web两个包，用全名是spring-boot-starter-aop和spring-boot-starter-web:

 <dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-aop</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> </dependencies>

引用插件

Spring Boot还提供了插件，我们也将其引用进来：

 <build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> </plugin> </plugins> </build>

主函数

库引用完了，我们就写一个主程序吧。按照惯例，我们将其保存在src/main/java目录下：

package cn.alios.system.service.prefix;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.ResponseBody;@SpringBootApplication@RequestMapping("/")public class Prefix { @RequestMapping("/") @ResponseBody public String home(){ return "Hello, Java Web World!"; } public static void main(String[] args) throws Exception{ SpringApplication.run(Prefix.class,args); }}

编译

下面我们用mvn package命令来编译生成可运行的jar包：

mvn package

输出类似于下面这样：

[INFO] Scanning for projects...[INFO] [INFO] ---------------< cn.alios.system.service.prefix:Prefix >----------------[INFO] Building Prefix 1.0.0-SNAPSHOT[INFO] --------------------------------[ jar ]---------------------------------[INFO] [INFO] --- maven-resources-plugin:3.1.0:resources (default-resources) @ Prefix ---[INFO] Using 'UTF-8' encoding to copy filtered resources.[INFO] Copying 0 resource[INFO] Copying 0 resource[INFO] [INFO] --- maven-compiler-plugin:3.8.0:compile (default-compile) @ Prefix ---[INFO] Changes detected - recompiling the module![INFO] Compiling 1 source file to /Users/ziyingliuziying/working/gitlab/Prefix/target/classes[INFO] [INFO] --- maven-resources-plugin:3.1.0:testResources (default-testResources) @ Prefix ---[INFO] Using 'UTF-8' encoding to copy filtered resources.[INFO] skip non existing resourceDirectory /Users/ziyingliuziying/working/gitlab/Prefix/src/test/resources[INFO] [INFO] --- maven-compiler-plugin:3.8.0:testCompile (default-testCompile) @ Prefix ---[INFO] Nothing to compile - all classes are up to date[INFO] [INFO] --- maven-surefire-plugin:2.22.1:test (default-test) @ Prefix ---[INFO] No tests to run.[INFO] [INFO] --- maven-jar-plugin:3.1.1:jar (default-jar) @ Prefix ---[INFO] Building jar: /Users/ziyingliuziying/working/gitlab/Prefix/target/Prefix-1.0.0-SNAPSHOT.jar[INFO] [INFO] --- spring-boot-maven-plugin:2.1.2.RELEASE:repackage (repackage) @ Prefix ---[INFO] Replacing main artifact with repackaged archive[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time: 2.462 s[INFO] Finished at: 2019-01-31T16:53:48+08:00[INFO] ------------------------------------------------------------------------

最后生成的包是
target/Prefix-1.0.0-SNAPSHOT.jar。

运行

调用java -jar
target/Prefix-1.0.0-SNAPSHOT.jar命令，运行这个Java程序，输出如下：

 . ____ _ __ _ _ /\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \/ ___)| |_)| | | | | || (_| | ) ) ) ) ' |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot :: (v2.1.2.RELEASE)2019-01-31 16:59:43.144 INFO 95879 --- [ main] cn.alios.system.service.prefix.Prefix : Starting Prefix v1.0.0-SNAPSHOT on ziyingliuziyingdeMacBook-Pro.local with PID 95879 (/Users/ziyingliuziying/working/gitlab/Prefix/target/Prefix-1.0.0-SNAPSHOT.jar started by ziyingliuziying in /Users/ziyingliuziying/working/gitlab/Prefix)2019-01-31 16:59:43.148 INFO 95879 --- [ main] cn.alios.system.service.prefix.Prefix : No active profile set, falling back to default profiles: default2019-01-31 16:59:44.289 INFO 95879 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port(s): 8080 (http)2019-01-31 16:59:44.325 INFO 95879 --- [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]2019-01-31 16:59:44.325 INFO 95879 --- [ main] org.apache.catalina.core.StandardEngine : Starting Servlet engine: [Apache Tomcat/9.0.14]2019-01-31 16:59:44.347 INFO 95879 --- [ main] o.a.catalina.core.AprLifecycleListener : The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [/Users/ziyingliuziying/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.]2019-01-31 16:59:44.435 INFO 95879 --- [ main] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext2019-01-31 16:59:44.435 INFO 95879 --- [ main] o.s.web.context.ContextLoader : Root WebApplicationContext: initialization completed in 1234 ms2019-01-31 16:59:44.665 INFO 95879 --- [ main] o.s.s.concurrent.ThreadPoolTaskExecutor : Initializing ExecutorService 'applicationTaskExecutor'2019-01-31 16:59:44.886 INFO 95879 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port(s): 8080 (http) with context path ''2019-01-31 16:59:44.889 INFO 95879 --- [ main] cn.alios.system.service.prefix.Prefix : Started Prefix in 2.161 seconds (JVM running for 2.561)

我们可以看到，启动了一个9.0.14版本的Apache Tomcat服务器，在8080端口上监听。

我们打开浏览器，访问http://127.0.0.1:8080/，可以看到『Hello, Java Web World!』这个字符串被显示出来。

再写一个Controller

在主函数里面可以处理请求，那么再其它类里面该如何做呢？我们通过写@Controller注解，加上@RequestMapping来指定路径，就可以了。

我们来写个例子：

package cn.alios.system.service.prefix.controller;import org.springframework.stereotype.Controller;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.ResponseBody;@Controller@RequestMapping("/test")public class TestController { @RequestMapping("/") @ResponseBody public String test(){ return "Test Controller!"; }}

还是mvn package，然后java -jar java -jar
target/Prefix-1.0.0-SNAPSHOT.jar。

在浏览器里试下
http://127.0.0.1:8080/test/，显示：『Test Controller!』

大功告成！现在整个从接收输入到显示输出的通道已经打通，是不是很easy?

作者：lusing