全民学后端快餐教程(1) - 只不过是写个Java程序

女爷i 2022-01-16 09:17 125阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

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: 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/][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/][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?

[原文链接][Link 1]

转载于:https://my.oschina.net/u/1464083/blog/3008039

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220114/993e6a7bcd7d4e5d838ccf423472bc32.png
[http_127.0.0.1_8080]: http://127.0.0.1:8080/
[http_127.0.0.1_8080_test]: http://127.0.0.1:8080/test/
[Link 1]: https://yq.aliyun.com/articles/689532?utm_content=g_1000040678