Spring-boot : Problem deploying WAR file in Tomcat 7 - spring-boot

Question

All,
I built a sample app using spring-boot, maven and it worked fine in my IDE using mvn spring-boot:run. But, when I attempt to deploy the app as a WAR(mvn clean install) in Tomcat 7 I get the following error. Please help me find the reason for the issue.
**INFO: validateJarFile(C:\_tools\apache-tomcat-7.0.96\webapps\spring-server-2.1.8.RELEASE\WEB-INF\lib\tomcat-embed-el-9.0.24.jar) - jar not loaded. See Servlet Spec 3.0, section 10.7.2. Offending class: javax/el/Expression.class Oct 16, 2019 2:31:57 PM org.apache.catalina.core.ContainerBase addChildInternal
SEVERE: ContainerBase.addChild: start:
org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/spring-server-2.1.8.RELEASE]]
at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:162)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:1018)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:994)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:662)
at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:1127)
at org.apache.catalina.startup.HostConfig$DeployWar.run(HostConfig.java:2020)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)**
Here's my POM file.
<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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.1.8.RELEASE</version>
<relativePath/> <!-- lookup parent from repository -->
</parent>
<groupId>com.service.data</groupId>
<artifactId>spring-server</artifactId>
<packaging>war</packaging>
<name>dev-tools-server</name>
<description>Spring Backend to access Tandem SQL/MX</description>
<properties>
<java.version>1.8</java.version>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
<scope>provided</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>
And this is my class file
#SpringBootApplication
public class SpringServerApplication extends SpringBootServletInitializer {
#Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
return application.sources(SpringServerApplication.class);
}
public static void main(String[] args) {
SpringApplication.run(SpringServerApplication.class, args);
}
}

Answer 1

Please, in your pom.xml, try to remove this:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
<scope>provided</scope>
</dependency>
And replace this:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
by this:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<exclusions>
<exclusion>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
</exclusion>
</exclusions>
</dependency>
And try again.
This makes sure that you're excluding the embedded Tomcat from the dependency tree in your project.
UPDATE
Add the Servet API as provided to your classpath.
The error in the coments happens because the tomcat (which has the servlet API implementation and specification as transitive dependency) has been removed, so there's no Servlet API available on your classpath
Add the 3.0.1 Servlet API specification as you're using Tomcat 7
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>3.0.1</version>
<scope>provided</scope>
</dependency>

Answer 2

You cannot run a Spring Boot 2.x application on Tomcat 7. Due to the simple fact that Spring Boot 2.x depends on Spring Framework 5.x which has a minimal Servlet API requirement for version 3.1 (see the Whats New In Spring 5).
You need at least Tomact 8.5 to run it (8.0 in theory but that has been superseded by 8.5).
To fix use Tomcat 8.5 or downgrade to the Spring Boot 1.5 line.
See also: Using Spring Boot 2.0 with Tomcat 7.0.82