如何在Java中读取扩展文件属性？

Question

我有扩展属性的文件 user.MYATTR

当我运行命令：时getfattr fileName -d，我得到了：

user.MYATTR="attribute_value"

我的操作系统是Red Hat Enterprise Linux Server 6.4版（圣地亚哥）

但是当我运行我的Java代码时

    LOG.debug("readExtentedAttribute(path={},name={}", path, name);

    UserDefinedFileAttributeView view = Files.getFileAttributeView(path, UserDefinedFileAttributeView.class);

    ByteBuffer buffer = ByteBuffer.allocate(view.size(name));
    view.read(name, buffer);
    buffer.flip();
    String value = Charset.defaultCharset().decode(buffer).toString();
    LOG.trace("extended attribute value = {}", value);

我正在例外

java.nio.file.FileSystemException: /opt/myfolder/myfile: Unable to get size of extended attribute 'user.MYATTR': No data available
    at sun.nio.fs.LinuxUserDefinedFileAttributeView.size(LinuxUserDefinedFileAttributeView.java:141) ~[na:1.7.0_25]
    at sk.tempest.cda.impex.service.FileServiceImpl.readExtentedAttribute(FileServiceImpl.java:39) ~[impex-impl-0.0.1-SNAPSHOT.jar!/:na]
    at sk.tempest.cda.impex.service.TapeMediumServiceImpl.hasDissemFlag(TapeMediumServiceImpl.java:189) ~[impex-impl-0.0.1-SNAPSHOT.jar!/:na]
    at sun.reflect.GeneratedMethodAccessor35.invoke(Unknown Source) ~[na:na]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_25]
    at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_25]
    at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:317) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:201) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at com.sun.proxy.$Proxy85.hasDissemFlag(Unknown Source) ~[na:na]
    at sk.tempest.cda.impex.service.TapeStorageServiceImpl.registerNewMedia(TapeStorageServiceImpl.java:158) ~[impex-impl-0.0.1-SNAPSHOT.jar!/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.7.0_25]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:1.7.0_25]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_25]
    at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_25]
    at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:317) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:190) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:98) ~[spring-tx-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:262) ~[spring-tx-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:95) ~[spring-tx-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:207) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at com.sun.proxy.$Proxy87.registerNewMedia(Unknown Source) ~[na:na]
    at sk.tempest.cda.impex.service.WorkerServiceImpl.checkNewMedia(WorkerServiceImpl.java:46) ~[impex-impl-0.0.1-SNAPSHOT.jar!/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.7.0_25]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:1.7.0_25]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_25]
    at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_25]
    at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:317) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:190) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:98) ~[spring-tx-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:262) ~[spring-tx-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:95) ~[spring-tx-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at org.springframework.aop.interceptor.AsyncExecutionInterceptor$1.call(AsyncExecutionInterceptor.java:97) ~[spring-aop-4.0.3.RELEASE.jar!/:4.0.3.RELEASE]
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334) ~[na:1.7.0_25]
    at java.util.concurrent.FutureTask.run(FutureTask.java:166) ~[na:1.7.0_25]
    at java.lang.Thread.run(Thread.java:724) ~[na:1.7.0_25]

Answer 1

最终我发现，当我想读取属性user.MYATTR时，必须使用名称MYATTR。

我只想提及我发现的有趣行为，这可能会导致误解，我想警告您:)

我的文件具有以下两个属性：

user.MYATTR1 user.somethingElse.MYATTR2

当我使用view.list（）方法列出属性时，我只看到了一个（没有用户）：MYATTR1

当我想读取属性值时，我必须使用不带'user。'的属性名称，因此对于提到的属性，它是：MYATTR1或somethingElse.MYATTR2