全民学后端快餐教程-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。

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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:

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">

4.0.0

cn.alios.system.service.prefix

Prefix

1.0.0-SNAPSHOT

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:

org.springframework.boot

spring-boot-starter-parent

2.1.2.RELEASE

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:

org.springframework.boot

spring-boot-starter-aop

org.springframework.boot

spring-boot-starter-web

引用插件

Spring Boot还提供了插件,我们也将其引用进来:

org.springframework.boot

spring-boot-maven-plugin

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: default

2019-01-31 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 WebApplicationContext

2019-01-31 16:59:44.435 INFO 95879 --- [ main] o.s.web.context.ContextLoader : Root WebApplicationContext: initialization completed in 1234 ms

2019-01-31 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?

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