Spring Boot Uber-Fat Jar运行失败：主类找不到问题求助

问题描述 #

为Spring Boot应用使用maven-shade-plugin构建Uber-Fat Jar，执行mvn clean install后生成普通Jar和Shaded Jar，普通Jar可正常运行，但运行Shaded Jar时抛出错误：

Error: Could not find or load main class com.walmart.SpringAppInitializer

pom.xml中插件配置如下：

<properties>
    <start-class>com.walmart.SpringAppInitializer</start-class>
</properties>

<plugins>
    <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
        <version>${spring-boot.version}</version>
        <configuration>
            <fork>true</fork>
            <mainClass>${start-class}</mainClass>
        </configuration>
        <executions>
            <execution>
                <phase>package</phase>
                <goals>
                    <goal>repackage</goal>
                </goals>
            </execution>
        </executions>
    </plugin>

    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-shade-plugin</artifactId>
        <version>3.3.0</version>
        <executions>
            <execution>
                <phase>package</phase>
                <goals>
                    <goal>shade</goal>
                </goals>
                <configuration>
                    <transformers>
                        <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                            <mainClass>${start-class}</mainClass>
                        </transformer>
                    </transformers>
                    <filters>
                        <filter>
                            <artifact>*:*</artifact>
                            <excludes>
                                <exclude>META-INF/maven/**</exclude>
                                <exclude>META-INF/*.SF</exclude>
                                <exclude>META-INF/*.DSA</exclude>
                                <exclude>META-INF/*.RSA</exclude>
                            </excludes>
                        </filter>
                    </filters>
                    <relocations>
                        <relocation>
                            <pattern>com.google.gson</pattern>
                            <shadedPattern>com.shaded.google.gson</shadedPattern>
                        </relocation>
                    </relocations>
                    <shadedArtifactAttached>true</shadedArtifactAttached>
                    <shadedClassifierName>shaded</shadedClassifierName>
                </configuration>
            </execution>
        </executions>
    </plugin>
</plugins>

核心原因分析 #

1. Spring Boot可执行Jar结构未被正确处理：Spring Boot的可执行Jar依赖特定的类加载器和资源结构，仅用ManifestResourceTransformer无法满足需求，会导致主类无法被Spring Boot的启动逻辑加载。
2. 插件执行顺序冲突：spring-boot-maven-plugin的repackage目标会修改Jar结构，若maven-shade-plugin在其之后执行，会破坏Spring Boot的可执行结构。

解决方案 #

方案1：为Shade插件添加Spring Boot专属转换器 #

通过引入Spring Boot提供的资源转换器，确保Shaded Jar符合Spring Boot的可执行规范：

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-shade-plugin</artifactId>
    <version>3.3.0</version>
    <executions>
        <execution>
            <phase>package</phase>
            <goals>
                <goal>shade</goal>
            </goals>
            <configuration>
                <transformers>
                    <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                        <mainClass>${start-class}</mainClass>
                    </transformer>
                    <!-- 合并Spring Boot配置文件 -->
                    <transformer implementation="org.springframework.boot.maven.PropertiesMergingResourceTransformer"/>
                    <!-- 处理Spring Boot可执行Jar结构 -->
                    <transformer implementation="org.springframework.boot.maven.SpringBootResourceTransformer"/>
                </transformers>
                <filters>
                    <filter>
                        <artifact>*:*</artifact>
                        <excludes>
                            <exclude>META-INF/maven/**</exclude>
                            <exclude>META-INF/*.SF</exclude>
                            <exclude>META-INF/*.DSA</exclude>
                            <exclude>META-INF/*.RSA</exclude>
                        </excludes>
                    </filter>
                </filters>
                <relocations>
                    <relocation>
                        <pattern>com.google.gson</pattern>
                        <shadedPattern>com.shaded.google.gson</shadedPattern>
                    </relocation>
                </relocations>
                <shadedArtifactAttached>true</shadedArtifactAttached>
                <shadedClassifierName>shaded</shadedClassifierName>
            </configuration>
        </execution>
    </executions>
    <!-- 引入依赖确保转换器可用 -->
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <version>${spring-boot.version}</version>
        </dependency>
    </dependencies>
</plugin>

方案2：调整插件执行顺序，让Spring Boot插件处理Shaded Jar #

先让maven-shade-plugin生成Uber Jar，再让spring-boot-maven-plugin对其执行repackage：

1. 修改Shade插件的执行阶段为prepare-package：

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-shade-plugin</artifactId>
    <version>3.3.0</version>
    <executions>
        <execution>
            <phase>prepare-package</phase>
            <goals>
                <goal>shade</goal>
            </goals>
            <configuration>
                <filters>
                    <filter>
                        <artifact>*:*</artifact>
                        <excludes>
                            <exclude>META-INF/maven/**</exclude>
                            <exclude>META-INF/*.SF</exclude>
                            <exclude>META-INF/*.DSA</exclude>
                            <exclude>META-INF/*.RSA</exclude>
                        </excludes>
                    </filter>
                </filters>
                <relocations>
                    <relocation>
                        <pattern>com.google.gson</pattern>
                        <shadedPattern>com.shaded.google.gson</shadedPattern>
                    </relocation>
                </relocations>
                <shadedArtifactAttached>true</shadedArtifactAttached>
                <shadedClassifierName>shaded</shadedClassifierName>
            </configuration>
        </execution>
    </executions>
</plugin>

2. 修改Spring Boot插件，指定处理带shaded分类器的Jar：

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <version>${spring-boot.version}</version>
    <configuration>
        <fork>true</fork>
        <mainClass>${start-class}</mainClass>
        <classifier>shaded</classifier>
    </configuration>
    <executions>
        <execution>
            <phase>package</phase>
            <goals>
                <goal>repackage</goal>
            </goals>
        </execution>
    </executions>
</plugin>

验证步骤 #

1. 确认主类已被包含：

jar tf your-app-shaded.jar | grep com/walmart/SpringAppInitializer.class

2. 查看Manifest配置是否正确：

jar xf your-app-shaded.jar META-INF/MANIFEST.MF
cat META-INF/MANIFEST.MF

内容的提问来源于stack exchange，提问作者amit kumar