深入理解Java中资源加载的方法及Spring的ResourceLoader应用

在Java开发中，资源加载是一个基础而重要的操作。本文将深入探讨Java中两种常见的资源加载方式：ClassLoader的getResource方法和Class的getResource方法，并介绍Spring框架中的ResourceLoader的应用。

1. 资源加载的两种方式

1.1 ClassLoader的getResource方法

URL resourceFromClass = classLoader.getResource("demo.txt");

ClassLoader的getResource方法是通过当前类的ClassLoader实例来加载资源。路径是相对于classpath的，可以轻松获取单个资源。需要注意的是，路径不要以"/"开头，否则可能返回null。

1.2 Class的getResource方法

URL resourceFromClassWithPath = aClass.getResource("dir/demo.txt");

Class的getResource方法通过当前类来加载资源，路径是相对于当前类所在的包。如果资源名以"/"开头，会以classpath为基准路径加载资源。这种方式可以方便地获取当前类包内的资源。

2. 获取批量资源

Enumeration<URL> resources = classLoader.getResources("META-INF/MANIFEST.MF");

通过ClassLoader的getResources方法，可以获取指定路径下的所有资源，返回一个Enumeration对象。这对于需要处理多个配置文件或元数据文件的场景非常有用。

3. Spring的ResourceLoader和ResourcePatternResolver

ResourcePatternResolver resolver = new PathMatchingResourcePatternResolver();
Resource[] springResources = resolver.getResources("classpath:/spring/*.xml");

Spring框架提供了ResourceLoader和ResourcePatternResolver接口，封装了资源加载的方法。在上述代码中，通过PathMatchingResourcePatternResolver实例，可以使用通配符获取classpath下的所有xml文件。这对于加载多个配置文件非常方便。

4. 完整示例

package com.lfsun.resources;

import org.springframework.core.io.Resource;
import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
import org.springframework.core.io.support.ResourcePatternResolver;

import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.Enumeration;

public class MyResource {

    public static void main(String[] args) throws Exception {
        // 获取单个资源
        // 获取当前类的Class对象
        Class<MyResource> aClass = MyResource.class;
        // 获取当前类的ClassLoader
        ClassLoader classLoader = aClass.getClassLoader();

        // 使用ClassLoader获取资源，路径相对于classpath
        URL resourceFromClass = classLoader.getResource("demo.txt");
        // 使用Class获取资源，路径相对于当前类所在的包
        URL resourceFromClassWithPath = aClass.getResource("dir/demo.txt");

        // 输出资源内容
        if (resourceFromClass != null) {
            byte[] bytes = readBytesFromStream(resourceFromClass.openStream());
            System.out.println("ClassLoader获取的资源内容: " + new String(bytes));
        }

        if (resourceFromClassWithPath != null) {
            byte[] bytes = readBytesFromStream(resourceFromClassWithPath.openStream());
            System.out.println("Class获取的资源内容: " + new String(bytes));
        }

        // 获取批量资源
        Enumeration<URL> resources = classLoader.getResources("META-INF/MANIFEST.MF");
        while (resources.hasMoreElements()) {
            URL manifestResource = resources.nextElement();
            System.out.println("Manifest资源: " + manifestResource);
        }

        // 使用Spring的ResourceLoader和ResourcePatternResolver
        ResourcePatternResolver resolver = new PathMatchingResourcePatternResolver();
        // 使用通配符获取classpath下的所有xml文件
        Resource[] springResources = resolver.getResources("classpath:/spring/*.xml");

        // 输出Spring资源内容
        for (Resource springResource : springResources) {
            InputStream stream = springResource.getInputStream();
            byte[] bytes = readBytesFromStream(stream);
            System.out.println("Spring资源内容: " + new String(bytes));
        }
    }

    private static byte[] readBytesFromStream(InputStream stream) throws IOException {
        StringBuilder buffer = new StringBuilder();
        byte[] bytes = new byte[1024];
        int bytesRead;
        while ((bytesRead = stream.read(bytes)) != -1) {
            buffer.append(new String(bytes, 0, bytesRead));
        }
        return buffer.toString().getBytes();
    }
}

输出：

D:\Config\java-17\bin\java.exe "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA 2023.2.4\lib\idea_rt.jar=62858:D:\Program Files\JetBrains\IntelliJ IDEA 2023.2.4\bin" -Dfile.encoding=UTF-8 -classpath D:\Users\Administrator\IdeaProjects\lfsun-study-java17\lfsun-study-resources\target\classes;D:\Users\Administrator\.m2\repository\org\springframework\boot\spring-boot-starter\3.2.1\spring-boot-starter-3.2.1.jar;D:\Users\Administrator\.m2\repository\org\springframework\boot\spring-boot\3.2.1\spring-boot-3.2.1.jar;D:\Users\Administrator\.m2\repository\org\springframework\spring-context\6.1.2\spring-context-6.1.2.jar;D:\Users\Administrator\.m2\repository\org\springframework\spring-aop\6.1.2\spring-aop-6.1.2.jar;D:\Users\Administrator\.m2\repository\org\springframework\spring-beans\6.1.2\spring-beans-6.1.2.jar;D:\Users\Administrator\.m2\repository\org\springframework\spring-expression\6.1.2\spring-expression-6.1.2.jar;D:\Users\Administrator\.m2\repository\io\micrometer\micrometer-observation\1.12.1\micrometer-observation-1.12.1.jar;D:\Users\Administrator\.m2\repository\io\micrometer\micrometer-commons\1.12.1\micrometer-commons-1.12.1.jar;D:\Users\Administrator\.m2\repository\org\springframework\boot\spring-boot-autoconfigure\3.2.1\spring-boot-autoconfigure-3.2.1.jar;D:\Users\Administrator\.m2\repository\org\springframework\boot\spring-boot-starter-logging\3.2.1\spring-boot-starter-logging-3.2.1.jar;D:\Users\Administrator\.m2\repository\ch\qos\logback\logback-classic\1.4.14\logback-classic-1.4.14.jar;D:\Users\Administrator\.m2\repository\ch\qos\logback\logback-core\1.4.14\logback-core-1.4.14.jar;D:\Users\Administrator\.m2\repository\org\apache\logging\log4j\log4j-to-slf4j\2.21.1\log4j-to-slf4j-2.21.1.jar;D:\Users\Administrator\.m2\repository\org\apache\logging\log4j\log4j-api\2.21.1\log4j-api-2.21.1.jar;D:\Users\Administrator\.m2\repository\org\slf4j\jul-to-slf4j\2.0.9\jul-to-slf4j-2.0.9.jar;D:\Users\Administrator\.m2\repository\jakarta\annotation\jakarta.annotation-api\2.1.1\jakarta.annotation-api-2.1.1.jar;D:\Users\Administrator\.m2\repository\org\springframework\spring-core\6.1.2\spring-core-6.1.2.jar;D:\Users\Administrator\.m2\repository\org\springframework\spring-jcl\6.1.2\spring-jcl-6.1.2.jar;D:\Users\Administrator\.m2\repository\org\yaml\snakeyaml\2.2\snakeyaml-2.2.jar;D:\Users\Administrator\.m2\repository\org\slf4j\slf4j-api\2.0.9\slf4j-api-2.0.9.jar com.lfsun.resources.MyResource
ClassLoader获取的资源内容: 这是 demo.txt 的内容。

Class获取的资源内容: 这是 dir/demo.txt 的内容。

Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/boot/spring-boot-starter/3.2.1/spring-boot-starter-3.2.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/boot/spring-boot/3.2.1/spring-boot-3.2.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/spring-context/6.1.2/spring-context-6.1.2.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/spring-aop/6.1.2/spring-aop-6.1.2.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/spring-beans/6.1.2/spring-beans-6.1.2.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/spring-expression/6.1.2/spring-expression-6.1.2.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/io/micrometer/micrometer-observation/1.12.1/micrometer-observation-1.12.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/io/micrometer/micrometer-commons/1.12.1/micrometer-commons-1.12.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/3.2.1/spring-boot-autoconfigure-3.2.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/boot/spring-boot-starter-logging/3.2.1/spring-boot-starter-logging-3.2.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/ch/qos/logback/logback-classic/1.4.14/logback-classic-1.4.14.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/ch/qos/logback/logback-core/1.4.14/logback-core-1.4.14.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/apache/logging/log4j/log4j-to-slf4j/2.21.1/log4j-to-slf4j-2.21.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/apache/logging/log4j/log4j-api/2.21.1/log4j-api-2.21.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/slf4j/jul-to-slf4j/2.0.9/jul-to-slf4j-2.0.9.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/jakarta/annotation/jakarta.annotation-api/2.1.1/jakarta.annotation-api-2.1.1.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/spring-core/6.1.2/spring-core-6.1.2.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/springframework/spring-jcl/6.1.2/spring-jcl-6.1.2.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/yaml/snakeyaml/2.2/snakeyaml-2.2.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Users/Administrator/.m2/repository/org/slf4j/slf4j-api/2.0.9/slf4j-api-2.0.9.jar!/META-INF/MANIFEST.MF
Manifest资源: jar:file:/D:/Program%20Files/JetBrains/IntelliJ%20IDEA%202023.2.4/lib/idea_rt.jar!/META-INF/MANIFEST.MF
Spring资源内容: <!-- config1.xml -->
<beans>
    <bean id="bean1" class="com.lfsun.resources.entity.Bean1"/>
</beans>

Spring资源内容: <!-- config2.xml -->
<beans>
    <bean id="bean2" class="com.lfsun.resources.entity.Bean2"/>
</beans>


Process finished with exit code 0

代码结构：

以上输出演示了通过ClassLoader和Class获取资源的内容，以及使用Spring的ResourceLoader加载多个资源的过程。