java:Apache Commons Configuration2占位符使用方式

${前缀:键名}

test.url=${classpath:cell-test.key}
test.runtime=${java:runtime}

public class ConfigurationReader {
    public ConfigurationReader() {
        // 创建CombinedConfiguration
        this.combinedConfig = new CombinedConfiguration();
        // 设置表达式引擎为默认表达式引擎
        this.combinedConfig.setExpressionEngine(DefaultExpressionEngine.instance());
        // 获取StringLookup对象用于变量插值
        InterpolatorStringLookup interpolator = StringLookupFactory.INSTANCE.interpolatorStringLookup();
        // 注册自定义的classpath前缀Lookup
        interpolator.registerLookup("classpath", ClasspathLookup.INSTANCE);
        // 为CombinedConfiguration设置变量插值器
        this.combinedConfig.setInterpolator(interpolator);
        // ...
    }
}

public class ClasspathLookup implements Lookup {
    public static final ClasspathLookup INSTANCE = new ClasspathLookup();
    
    @Override
    public Object lookup(String variable) {
        if(Strings.isNullOrEmpty(variable)) {
            return null;
        }
        URL resourceUrl = getClass().getClassLoader().getResource(variable);
        return (resourceUrl != null) ? resourceUrl : null;
    }
}

// 获取StringLookup对象用于变量插值
InterpolatorStringLookup interpolator = StringLookupFactory.INSTANCE.interpolatorStringLookup();
// 注册自定义的classpath前缀Lookup
interpolator.registerLookup("classpath", ClasspathLookup.INSTANCE);
// 为CombinedConfiguration设置变量插值器
this.combinedConfig.setInterpolator(interpolator);

# 在cell-config.properties中定义
test.url=${classpath:cell-test.key}
test.url2=${classpath:nofound.key}
test.runtime=${java:runtime}

@Test
public void test1ClasspathLookup() {
    // 读取通过classpath前缀定义的资源路径(URL)
    String value = config.getString("test.url");
    log("value:{}", value);
    assertNotNull(value);
    assertNotEquals("${classpath:cell-test.key}", value); // 验证占位符已被解析
    
    // 读取为URL类型
    URL urlValue = config.get(URL.class, "test.url");
    log("value:{}", urlValue);
    assertNotNull(urlValue);
    
    // 测试不存在的资源（应保留原始占位符）
    String notFound = config.getString("test.url2");
    log("value:{}", notFound);
    assertEquals("${classpath:nofound.key}", notFound); // 资源不存在时保持原样
}