使用java 11运行spring boot多版本jar时的ClassNotFoundException

Question

在尝试针对不同版本的java[Java8（默认）和Java11]运行spring boot应用程序（使用Multi-Release jar）时

当用Java8运行时，该项目运行良好。但是，在使用Java11运行应用程序时，将获得ClassNotFoundException

项目结构：

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.5.5</version>
    <relativePath/> 
</parent>
<groupId>com.example</groupId>
<artifactId>demo</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>demo</name>
<description>Demo project for Spring Boot</description>
<properties>
    <java.version>1.8</java.version>
    <start.class>com.example.demo.DemoApplication</start.class>
</properties>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>com.inq</groupId>
        <artifactId>touchsocial-stress-testing</artifactId>
        <version>4.0-SNAPSHOT</version>
        <scope>test</scope>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-toolchains-plugin</artifactId>
            <version>3.0.0</version>
            <configuration>
                <toolchains>
                    <jdk>
                        <version>zulu-11</version>
                    </jdk>
                </toolchains>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>toolchain</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.1</version>
            <executions>
                <execution>
                    <id>default-compile</id>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                    <configuration>
                        <release>8</release>
                    </configuration>
                </execution>
                <execution>
                    <id>compile-java-11</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                    <configuration>
                        <release>11</release>
                        <compileSourceRoots>
                            <compileSourceRoot>${project.basedir}/src/main/java</compileSourceRoot>
                        </compileSourceRoots>
                        <multiReleaseOutput>true</multiReleaseOutput>
                    </configuration>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <version>3.2.0</version>
            <configuration>
                <archive>
                    <manifestEntries>
                        <Multi-Release>true</Multi-Release>
                    </manifestEntries>
                </archive>
            </configuration>
        </plugin>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <mainClass>${start.class}</mainClass>
            </configuration>
        </plugin>
    </plugins>
</build>

注意：出于POC的目的，DemoApplication.java的内容完全相同

Answer 1

您的Maven配置将您的多版本类打包在JAR的根中。这意味着它们是由系统类加载器加载的。系统类加载器看不到boot-inf/lib中打包的任何依赖项，因此无法加载SpringApplication。

您需要将应用程序配置为在boot-inf/classes下面打包多版本类。一种方法是配置编译任务的输出目录：

<execution>
    <id>compile-java-11</id>
    <phase>compile</phase>
    <goals>
        <goal>compile</goal>
    </goals>
    <configuration>
        <release>11</release>
        <compileSourceRoots>
            <compileSourceRoot>${project.basedir}/src/main/java</compileSourceRoot>
        </compileSourceRoots>
        <outputDirectory>${project.build.outputDirectory}/BOOT-INF/classes/</outputDirectory>
        <multiReleaseOutput>true</multiReleaseOutput>
    </configuration>
</execution>

如果您在进行此更改后进行了一个干净的构建，那么Java 11特定的类现在将被打包在boot-inf/classes/meta-inf/versions/11/下面。因此，它将由Spring Boot的类加载器加载，并且能够访问boot-inf/lib中打包的JAR中的类。