java - findResource ("") 当 module-info.java 存在时返回 null，这是为什么呢？

Question

我正在调试为什么在我的 Spring Boot 应用程序中存在 module-info.java 时，spring-orm 在启动期间会引发异常。这是一个异常(exception):

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaConfiguration.class]: Invocation of init method failed; nested exception is java.lang.NoClassDefFoundError: javax/transaction/UserTransaction
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1699) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:573) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at spring.context@5.0.8.RELEASE/org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1089) ~[spring-context-5.0.8.RELEASE.jar:na]
    at spring.context@5.0.8.RELEASE/org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:859) ~[spring-context-5.0.8.RELEASE.jar:na]
    at spring.context@5.0.8.RELEASE/org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.0.8.RELEASE.jar:na]
    at spring.boot@2.0.4.RELEASE/org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:140) ~[spring-boot-2.0.4.RELEASE.jar:na]
    at spring.boot@2.0.4.RELEASE/org.springframework.boot.SpringApplication.refresh(SpringApplication.java:762) [spring-boot-2.0.4.RELEASE.jar:na]
    at spring.boot@2.0.4.RELEASE/org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:398) [spring-boot-2.0.4.RELEASE.jar:na]
    at spring.boot@2.0.4.RELEASE/org.springframework.boot.SpringApplication.run(SpringApplication.java:330) [spring-boot-2.0.4.RELEASE.jar:na]
    at spring.boot@2.0.4.RELEASE/org.springframework.boot.SpringApplication.run(SpringApplication.java:1258) [spring-boot-2.0.4.RELEASE.jar:na]
    at spring.boot@2.0.4.RELEASE/org.springframework.boot.SpringApplication.run(SpringApplication.java:1246) [spring-boot-2.0.4.RELEASE.jar:na]
    at tech.flexpoint.dashmanserver/tech.flexpoint.dashmanserver.DashmanServerApplication.main(DashmanServerApplication.java:13) [classes/:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:564) ~[na:na]
    at spring.boot.devtools@2.0.4.RELEASE/org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-2.0.4.RELEASE.jar:na]
Caused by: java.lang.NoClassDefFoundError: javax/transaction/UserTransaction
    at java.base/java.lang.Class.getDeclaredMethods0(Native Method) ~[na:na]
    at java.base/java.lang.Class.privateGetDeclaredMethods(Class.java:3119) ~[na:na]
    at java.base/java.lang.Class.privateGetPublicMethods(Class.java:3144) ~[na:na]
    at java.base/java.lang.Class.getMethods(Class.java:1863) ~[na:na]
    at hibernate.core@5.2.17.Final/org.hibernate.service.internal.AbstractServiceRegistryImpl.applyInjections(AbstractServiceRegistryImpl.java:288) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.service.internal.AbstractServiceRegistryImpl.injectDependencies(AbstractServiceRegistryImpl.java:279) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:239) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:210) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.service.internal.SessionFactoryServiceRegistryImpl.getService(SessionFactoryServiceRegistryImpl.java:80) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.internal.SessionFactoryImpl.canAccessTransactionManager(SessionFactoryImpl.java:942) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.internal.SessionFactoryImpl.buildCurrentSessionContext(SessionFactoryImpl.java:953) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:319) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.boot.internal.SessionFactoryBuilderImpl.build(SessionFactoryBuilderImpl.java:462) ~[hibernate-core-5.2.17.Final.jar:na]
    at hibernate.core@5.2.17.Final/org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:892) ~[hibernate-core-5.2.17.Final.jar:na]
    at spring.orm@5.0.8.RELEASE/org.springframework.orm.jpa.vendor.SpringHibernateJpaPersistenceProvider.createContainerEntityManagerFactory(SpringHibernateJpaPersistenceProvider.java:57) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at spring.orm@5.0.8.RELEASE/org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:365) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at spring.orm@5.0.8.RELEASE/org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.buildNativeEntityManagerFactory(AbstractEntityManagerFactoryBean.java:390) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at spring.orm@5.0.8.RELEASE/org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:377) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at spring.orm@5.0.8.RELEASE/org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.afterPropertiesSet(LocalContainerEntityManagerFactoryBean.java:341) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1758) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at spring.beans@5.0.8.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1695) ~[spring-beans-5.0.8.RELEASE.jar:na]
    ... 21 common frames omitted
Caused by: java.lang.ClassNotFoundException: javax.transaction.UserTransaction
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582) ~[na:na]
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:190) ~[na:na]
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:499) ~[na:na]
    ... 42 common frames omitted

如果 module-info.java 存在，我将问题追踪到 URLClassLoader.findResource("") 返回 null 但 "file:/C:/Users/pupeno/Documents/Dashman/code/dashmanserver/target/classes/" 如果不是。

我创建了引发相同异常的最小可能示例。要运行它，您需要:

1. 从此处克隆并安装 Moditect 的最新副本:https://github.com/moditect/moditect由于此错误修复尚未发布:https://github.com/moditect/moditect/issues/51
2. 从 https://github.com/dashmantech/demo 克隆演示代码库
3. 使用凭据 demo/confi/application.properties 设置本地 PostgreSQL 数据库
4. 先运行mvn clean package，ModiTec创建所有模块
5. 在最新的 IntelliJ 副本中打开项目
6. 单击“运行演示”配置文件的播放(.idea 目录包含在相应的运行配置文件中，带有参数等)。

我需要 findResource("") 来返回 "file:/C:/Users/pupeno/Documents/Dashman/code/dashmanserver/target/classes/"以便 spring-orm 可以工作。

findResource("") 看起来像这样:

public URL findResource(final String name) {
    /*
     * The same restriction to finding classes applies to resources
     */
    URL url = AccessController.doPrivileged(
        new PrivilegedAction<>() {
            public URL run() {
                return ucp.findResource(name, true);
            }
        }, acc);

    return url != null ? URLClassPath.checkURL(url) : null;
}

所以我可以看到在不使用模块系统的情况下可以进行一些访问，但是当存在 module-infe.java 时，Java 的模块系统会阻止它。我的问题是我不知道如何使它工作，应该导出或打开什么才能工作？

Spring Boot 导致该方法调用的方式是通过 RestartClassLoader，URLClassLoader 的子类，具体来说，第 124 行调用 super.findResource(名称) 在:

@Override
public URL findResource(String name) {
    final ClassLoaderFile file = this.updatedFiles.getFile(name);
    if (file == null) {
        return super.findResource(name);
    }
    if (file.getKind() == Kind.DELETED) {
        return null;
    }
    return AccessController
            .doPrivileged((PrivilegedAction<URL>) () -> createFileUrl(name, file));
}

正在使用的特定 RestartClassLoader 实例是 ClassPathResource 的成员，它是这样定义的:

this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());

在构造函数中，line 85 .

最后，getDefaultClassLoader() 看起来像这样:

/**
 * Return the default ClassLoader to use: typically the thread context
 * ClassLoader, if available; the ClassLoader that loaded the ClassUtils
 * class will be used as fallback.
 * <p>Call this method if you intend to use the thread context ClassLoader
 * in a scenario where you clearly prefer a non-null ClassLoader reference:
 * for example, for class path resource loading (but not necessarily for
 * {@code Class.forName}, which accepts a {@code null} ClassLoader
 * reference as well).
 * @return the default ClassLoader (only {@code null} if even the system
 * ClassLoader isn't accessible)
 * @see Thread#getContextClassLoader()
 * @see ClassLoader#getSystemClassLoader()
 */
@Nullable
public static ClassLoader getDefaultClassLoader() {
    ClassLoader cl = null;
    try {
        cl = Thread.currentThread().getContextClassLoader();
    }
    catch (Throwable ex) {
        // Cannot access thread context ClassLoader - falling back...
    }
    if (cl == null) {
        // No thread context class loader -> use class loader of this class.
        cl = ClassUtils.class.getClassLoader();
        if (cl == null) {
            // getClassLoader() returning null indicates the bootstrap ClassLoader
            try {
                cl = ClassLoader.getSystemClassLoader();
            }
            catch (Throwable ex) {
                // Cannot access system ClassLoader - oh well, maybe the caller can live with null...
            }
        }
    }
    return cl;
}

我的 module-info.java 包含:

module tech.flexpoint.dashman {
    exports tech.flexpoint.dashman to com.fasterxml.jackson.databind;
    exports tech.flexpoint.dashman.controllers.configurator to javafx.fxml;

    opens tech.flexpoint.dashman to javafx.graphics, jna;
    opens tech.flexpoint.dashman.controllers.common to javafx.fxml;
    opens tech.flexpoint.dashman.controllers.configurator to javafx.fxml;
    opens tech.flexpoint.dashman.models to org.hibernate.validator, tech.flexpoint.dashmancommon, javafx.base;

    opens common;
    opens configurator;
    opens displayer;
    opens winscreensaver;

    requires appdirs;
    requires org.bouncycastle.provider;
    requires com.fasterxml.jackson.core;
    requires com.fasterxml.jackson.databind;
    requires com.fasterxml.jackson.datatype.jdk8;
    requires io.sentry;
    requires jackson.annotations;
    requires java.desktop;
    requires java.sql;
    requires java.validation;
    requires javafx.controls;
    requires javafx.fxml;
    requires javafx.graphics;
    requires javafx.media;
    requires javafx.web;
    requires jna;
    requires jna.platform;
    requires org.apache.commons.lang3;
    requires org.kordamp.ikonli.javafx;
    requires org.kordamp.ikonli.fontawesome5;
    requires spring.core;
    requires spring.retry;
    requires spring.web;
    requires tech.flexpoint.dashmancommon;
}

在 IntelliJ 中我启用了这些插件:

• Lombok 插件
• .ginore
• PowerShell
• VisualVM 启动器
• ANSI 荧光笔
• 批处理脚本支持
• 字节码查看器
• CMD 支持
• 版权所有
• 覆盖范围
• CSS 支持
• 数据库工具和 SQL
• Git 集成
• GitHub
• 分级
• 时髦的
• Heroku 集成
• HTML 工具
• HTTP 客户端
• 用于 Java 的 l18n
• IDE 设置同步
• Java 字节码反编译器
• Java EE:EJB、JPA、Servlet
• Java 流调试器
• JavaFX
• JUnit
• 行排序器
• Markdown 支持
• Maven 集成
• Maven 集成扩展
• 持久性框架支持
• 属性支持
• Smali 支持
• Spring AOP/@AspectJ
• Spring 批处理
• Spring 启动
• Spring 数据
• Spring 集成模式
• Spring OSGi
• Spring 安全性
• Spring 支持
• Spring Web 服务
• Spring WebSocket
• 终端
• YAML

Answer 1

我注意到的一件事是您的应用程序(假设它被打包在 tech.flexpoint.dashman 中)似乎没有以任何方式向 Spring 开放，这肯定会导致类失败加载/非法访问。

我希望在 module-info.java 中看到类似的内容(取决于您的 Spring 依赖项):

opens tech.flexpoint.dashman to spring.core, spring.beans, spring.context;

---

异常是一个NoClassDefFoundError，当一个在编译时已知的类的类定义无法解析时，在运行时抛出该异常，在这种情况下是接口(interface)javax.transaction.UserTransaction，它是 Java Transaction API (JTA) 的一部分。

正如其他人所指出的，JTA 没有与 JDK 捆绑在一起，需要作为编译依赖项添加。但是，需要加载 UserTransaction 类定义的类来自 spring-boot-autoconfigure 工件，该工件负责自己的依赖项( spring-boot-autoconfigure@2.4.0.RELEASE 🡒 jboss-transaction-spi@7.6.0.Final 🡒 jboss-transaction-api_1.2_spec@1.1.1.Final ) ，所以你应该不需要需要添加JTA作为依赖。

但是，因为您想将自己的应用程序打包为 Java 9 模块，所以它需要 explicitly state its dependencies 。 spring-boot-autoconfigure 还不是一个模块化的 Java 9 库，并且不会为您执行此操作(即传递性)。 JTA的自动模块名是java.transaction，所以需要在module-info.java中添加需求:

requires java.transaction;

---

我让您的示例运行，并且在从 IntelliJ IDEA 运行时确实得到了 NoClassDefFoundError。堆栈跟踪指向一个 ClassNotFoundException，它指示类路径问题。由于 IDEA 在从那里启动应用程序时会计算类路径，所以我想看看在使用 spring-boot-maven-plugin 运行应用程序时是否可以重现错误。

我将IDEA运行配置复制到spring-boot-maven-plugin配置中，如下图:

<plugin>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-maven-plugin</artifactId>
  <configuration>
      <mainClass>tech.flexpoint.demo.DemoApplication</mainClass>
      <jvmArguments>--show-module-resolution --add-opens=java.base/java.lang=spring.core --add-opens=java.base/java.io=tomcat.embed.core --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.rmi/sun.rmi.transport=ALL-UNNAMED</jvmArguments>
      <workingDirectory>${project.basedir}</workingDirectory>
  </configuration>
</plugin>

然后我调用 mvn spring-boot:run 瞧，应用程序成功启动，没有错误。我只能得出结论，这是 IntelliJ 计算的类路径的问题。