java.lang.ClassNotFoundException 怎么可能？

Question

我添加了具有此依赖项的库

        <dependency>
            <groupId>org.codehaus.jackson</groupId>
            <artifactId>jackson-core-asl</artifactId>
            <version>1.9.13</version>
        </dependency>
        <dependency>
            <groupId>org.codehaus.jackson</groupId>
            <artifactId>jackson-mapper-asl</artifactId>
            <version>1.9.13</version>
        </dependency>

然后当我尝试创建 pojo 类 Idea 告诉我它找不到类但类存在java.lang.ClassNotFoundException:org.codehaus.jackson.annotate.JsonUnwrapped

@Override
public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    resp.addHeader("Content-Type", "application/json; charset=utf-8");
    StringBuilder builder = new StringBuilder();
    String a = null;
    try (BufferedReader reader = new BufferedReader(new InputStreamReader(req.getInputStream(), "UTF-8"))){
        while ((a = reader.readLine())!=null){
            builder.append(a);
        }
    }
    ObjectMapper objectMapper = new ObjectMapper();
    UserPojo pojo = objectMapper.readValue(builder.toString(),UserPojo.class);
   }

示例图片:

我在类路径中有这个库。我尝试使用 jackson 的另一个版本。当我使用版本 2.+ 时，我遇到了另一个疯狂的错误 Idea can't find class ObjectMapper only in this class。

在这张图片上我有类(ObjectMapper)未找到错误:

在这张图片上我没有错误:

但这两个类在一个包中。这是怎么可能的以及如何解决？

添加pom

<?xml version="1.0" encoding="UTF-8"?>
<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>PetClinic</groupId>
    <artifactId>cpw</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>war</packaging>

    <name>Clinic Pet Web</name>
    <url>http://maven.apache.org</url>

    <dependencies>

        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>5.3.5.Final</version>
        </dependency>
        <dependency>
            <groupId>postgresql</groupId>
            <artifactId>postgresql</artifactId>
            <version>9.1-901-1.jdbc4</version>
        </dependency>
        <dependency>
            <groupId>org.codehaus.jackson</groupId>
            <artifactId>jackson-core-asl</artifactId>
            <version>1.9.13</version>
        </dependency>
        <dependency>
            <groupId>org.codehaus.jackson</groupId>
            <artifactId>jackson-mapper-asl</artifactId>
            <version>1.9.13</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
        <dependency>
            <groupId>jstl</groupId>
            <artifactId>jstl</artifactId>
            <version>1.2</version>
        </dependency>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.5</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>commons-logging</groupId>
            <artifactId>commons-logging</artifactId>
            <version>1.1.1</version>
        </dependency>
        <dependency>
            <groupId>taglibs</groupId>
            <artifactId>standard</artifactId>
            <version>1.1.2</version>
        </dependency>
        <dependency>
            <groupId>commons-lang</groupId>
            <artifactId>commons-lang</artifactId>
            <version>2.1</version>
        </dependency>
        <dependency>
            <groupId>org.codehaus.plexus</groupId>
            <artifactId>plexus-utils</artifactId>
            <version>1.1</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.8.2</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>PetClinic</groupId>
            <artifactId>se</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-all</artifactId>
            <version>1.10.19</version>
            <scope>test</scope>
        </dependency>

    </dependencies>

    <build>
        <finalName>PetClinicWeb</finalName>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.3</version>
                <configuration>
                    <source>1.7</source>
                    <target>1.7</target>
                    <encoding>UTF-8</encoding>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.jacoco</groupId>
                <artifactId>jacoco-maven-plugin</artifactId>
                <version>0.7.4.201502262128</version>
                <configuration>
                    <destFile>${basedir}/target/coverage-reports/jacoco-unit.exec</destFile>
                    <dataFile>${basedir}/target/coverage-reports/jacoco-unit.exec</dataFile>
                </configuration>
                <executions>
                    <execution>
                        <id>jacoco-initialize</id>
                        <goals>
                            <goal>prepare-agent</goal>
                        </goals>
                    </execution>
                    <execution>
                        <id>jacoco-site</id>
                        <phase>package</phase>
                        <goals>
                            <goal>report</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

Answer 1

尝试使用 com.fasterxml jackson-annotations 和 jackson-core 包，一般情况下最好使用 2+ 版本。似乎您拥有的 JsonUnwrapped 类来自版本 1，位于 org.codehaus.jackson 包中，而其他一些库使用版本 2(具有不同的包 - com.fasterxml.jackson)。尝试用以下内容替换您的 Jackson 依赖项:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-annotations</artifactId>
    <version>2.7.8</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.7.8</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.7.8</version>
</dependency>

至于缺少 ObjectMapper，您很可能没有更新导入语句。因此切换到版本 2 并用 fastxml 替换所有导入的 codehouse jackson。所有 Jackson 库拥有相同版本非常重要。

此外，如果您的项目有任何父项目，请检查该项目是否导入了 jackson 以及它的版本。它可能有旧版本，在这种情况下分别更改您的库版本，或者只是删除该版本，以便它将从父级继承。

参见https://dzone.com/articles/jackson-dependency-issue-in-spring-boot-with-maven了解更多详情