全民学后端快餐教程(1) – 只不过是写个Java程序

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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?
本文作者:lusing
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