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: 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?

本文作者：lusing

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