是因为你的依赖关系没有包含在最终的jar文件中。

看一看例子这里或 here 。 或者使用。 / gradlew clean build 应该自动将应用程序正确打包。查看官方指南

I have a spring boot project using gradle

apply plugin: 'java'
apply plugin: 'idea'
apply plugin: 'jetty'
apply plugin: 'war'
apply plugin: 'org.springframework.boot'

repositories {
    mavenCentral()
}

dependencies {
    compile("org.springframework.boot:spring-boot-starter-web:1.5.1.RELEASE") {
        exclude module: "spring-boot-starter-tomcat"
    }
    compile group: 'org.springframework.boot', name: 'spring-boot-starter-jetty', version: '1.5.1.RELEASE'
    compile group: 'org.springframework.boot', name: 'spring-boot-starter-actuator', version: '1.5.1.RELEASE'
    compile group: 'org.springframework', name: 'spring-webmvc', version: '4.3.6.RELEASE'
    compile group: 'org.springframework', name: 'spring-context-support', version: '4.3.6.RELEASE'
    compile group: 'org.springframework', name: 'spring-orm', version: '4.3.6.RELEASE'
    compile group: 'javax.servlet', name: 'javax.servlet-api', version: '3.1.0'
    compile group: 'org.freemarker', name: 'freemarker', version: '2.3.23'
    compile group: 'mysql', name: 'mysql-connector-java', version: '6.0.5'
    compile group: 'org.hibernate', name: 'hibernate-core', version: '5.1.4.Final'
    compile group: 'commons-validator', name: 'commons-validator', version: '1.5.1'
    compile group: 'junit', name: 'junit', version: '4.12'
    compile group: 'org.springframework.boot', name: 'spring-boot-starter-test', version: '1.5.1.RELEASE'
}

task wrapper(type: Wrapper) {
    gradleVersion = '2.2'
}

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:1.5.1.RELEASE")
    }
}

jar {
    baseName = 'base-name'
    version = '0.1.0'
    manifest {
        attributes 'Main-Class': 'some.application.Application'
    }
}

sourceCompatibility = 1.8
targetCompatibility = 1.8

I build using
./gradlew clean jar
Then try to run using
java -jar <jar-name>.jar

However I get an error saying

Exception in thread "main" java.lang.NoClassDefFoundError: org/springframework/boot/SpringApplication
        at some.application.Application.main(Application.java:11)
Caused by: java.lang.ClassNotFoundException: org.springframework.boot.SpringApplication
        at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        ... 1 more

I am so confused as to why I can run it in IDE but not as a jar. Also I found out that my jar doesn't contain any libraries at all.

EDIT:

This is my Application.class

package some.application;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application
{
    public static void main(String[] args)
    {
        SpringApplication.run(Application.class, args);
    }
}

This is because your dependencies are not included into the end jar file.

Take a look on the examples here or here.

Or alternatively use ./gradlew clean build that should automatically pack the app correctly. Take a look on the official guide