全民學後端快餐教程(1) - 只不過是寫個Java程序

全民學後端快餐教程(1) - 只不過是寫個Java程序

服務端程序本質上也只是個Java程序，它接收客戶端的輸入，然後將計算處理後的返回值返回給客戶端。下面我們就以這個思路開始Java後端之旅吧。

引用Spring Boot庫

處理HTTP請求之類的事情，我們需要庫的幫助。所以第一步我們就把Spring Boot引入進來。
不需要任何工具，我們使用maven來管理庫依賴，這樣我們只要寫一個pom.xml就好了。我們先寫一個最簡的pom.xml。主要是定義groupId，比如是我司，還有artifactId，就是應用的具體名字：

<?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

類似於類的繼承，我們不是從頭開發，而是繼承Spring Boot Starter框架。添加parent的內容如下：

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

2.1.2是本文寫作時，Spring Boot的最新版本。

增加依賴

爲了自動下載庫，我們將需要的幾個庫添加到pom.xml中的依賴項中。這樣maven就可以幫我們從倉庫中下載最新的庫代碼。
我們需要AOP和Web兩個包，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還提供了插件，我們也將其引用進來：

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

主函數

庫引用完了，我們就寫一個主程序吧。按照慣例，我們將其保存在src/main/java目錄下：

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);
    }
}

編譯

下面我們用mvn package命令來編譯生成可運行的jar包：

mvn package

輸出類似於下面這樣：

[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] ------------------------------------------------------------------------

最後生成的包是target/Prefix-1.0.0-SNAPSHOT.jar。

運行

調用java -jar target/Prefix-1.0.0-SNAPSHOT.jar命令，運行這個Java程序，輸出如下：

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: 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)

我們可以看到，啓動了一個9.0.14版本的Apache Tomcat服務器，在8080端口上監聽。

我們打開瀏覽器，訪問http://127.0.0.1:8080/，可以看到『Hello, Java Web World!』這個字符串被顯示出來。

再寫一個Controller

在主函數裏面可以處理請求，那麼再其它類裏面該如何做呢？我們通過寫@Controller註解，加上@RequestMapping來指定路徑，就可以了。
我們來寫個例子：

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!";
    }
}

還是mvn package，然後java -jar java -jar target/Prefix-1.0.0-SNAPSHOT.jar。
在瀏覽器裏試下http://127.0.0.1:8080/test/，顯示：『Test Controller!』
大功告成！現在整個從接收輸入到顯示輸出的通道已經打通，是不是很easy?