HDFS-6778. The extended attributes javadoc should simply refer to the user docs. Contributed by Charles Lamb.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1614547 13f79535-47bb-0310-9956-ffa450edef68

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/AbstractFileSystem.java

@@ -1040,21 +1040,10 @@ public abstract class AbstractFileSystem {
 
   /**
    * Set an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * A regular user can only set an xattr for the "user" namespace.
-   * The super user can set an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set when the logged-in user has the correct permissions.
-   * If the xattr exists, it will be replaced.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to modify
    * @param name xattr name.
@@ -1069,21 +1058,10 @@ public abstract class AbstractFileSystem {
 
   /**
    * Set an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
-   * <p/>
-   * A regular user can only set an xattr for the "user" namespace.
-   * The super user can set an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set when the logged-in user has the correct permissions.
-   * If the xattr exists, it will be replaced.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to modify
    * @param name xattr name.
@@ -1099,18 +1077,10 @@ public abstract class AbstractFileSystem {
 
   /**
    * Get an xattr for a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
-   * <p/>
-   * A regular user can only get an xattr for the "user" namespace.
-   * The super user can get an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * An xattr will only be returned when the logged-in user has the correct permissions.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attribute
    * @param name xattr name.
@@ -1127,13 +1097,7 @@ public abstract class AbstractFileSystem {
    * Only those xattrs for which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattrs for the "user" namespace.
-   * The super user can only get xattrs for "user" and "trusted" namespaces.
-   * The xattr of "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @return Map<String, byte[]> describing the XAttrs of the file or directory
@@ -1149,13 +1113,7 @@ public abstract class AbstractFileSystem {
    * Only those xattrs for which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattrs for the "user" namespace.
-   * The super user can only get xattrs for "user" and "trusted" namespaces.
-   * The xattr of "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @param names XAttr names.
@@ -1173,14 +1131,7 @@ public abstract class AbstractFileSystem {
    * Only the xattr names for which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattr names for the "user" namespace.
-   * The super user can only get xattr names for the "user" and "trusted"
-   * namespaces.
-   * The xattr names in the "security" and "system" namespaces are only
-   * used/exposed internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @return Map<String, byte[]> describing the XAttrs of the file or directory
@@ -1194,21 +1145,10 @@ public abstract class AbstractFileSystem {
 
   /**
    * Remove an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
-   * <p/>
-   * A regular user can only remove an xattr for the "user" namespace.
-   * The super user can remove an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set when the logged-in user has the correct permissions.
-   * If the xattr exists, it will be replaced.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to remove extended attribute
    * @param name xattr name

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FileContext.java

@@ -2297,21 +2297,10 @@ public final class FileContext {
 
   /**
    * Set an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * A regular user can only set an xattr for the "user" namespace.
-   * The super user can set an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set when the logged-in user has the correct permissions.
-   * If the xattr exists, it will be replaced.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to modify
    * @param name xattr name.
@@ -2326,21 +2315,10 @@ public final class FileContext {
 
   /**
    * Set an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
-   * <p/>
-   * A regular user can only set an xattr for the "user" namespace.
-   * The super user can set an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set when the logged-in user has the correct permissions.
-   * If the xattr exists, it will be replaced.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to modify
    * @param name xattr name.
@@ -2363,19 +2341,10 @@ public final class FileContext {
 
   /**
    * Get an xattr for a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
-   * <p/>
-   * 
-   * A regular user can only get an xattr for the "user" namespace.
-   * The super user can get an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * An xattr will only be returned when the logged-in user has the correct permissions.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attribute
    * @param name xattr name.
@@ -2398,13 +2367,7 @@ public final class FileContext {
    * Only those xattrs for which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattrs for the "user" namespace.
-   * The super user can only get xattrs for "user" and "trusted" namespaces.
-   * The xattr of "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @return Map<String, byte[]> describing the XAttrs of the file or directory
@@ -2426,13 +2389,7 @@ public final class FileContext {
    * Only those xattrs for which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattrs for the "user" namespace.
-   * The super user can only get xattrs for "user" and "trusted" namespaces.
-   * The xattr of "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @param names XAttr names.
@@ -2453,21 +2410,10 @@ public final class FileContext {
 
   /**
    * Remove an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * A regular user can only remove an xattr for the "user" namespace.
-   * The super user can remove an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set when the logged-in user has the correct permissions.
-   * If the xattr exists, it will be replaced.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to remove extended attribute
    * @param name xattr name
@@ -2490,14 +2436,7 @@ public final class FileContext {
    * Only those xattr names which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattr names for the "user" namespace.
-   * The super user can only get xattr names for "user" and "trusted"
-   * namespaces.
-   * The xattrs of the "security" and "system" namespaces are only
-   * used/exposed internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @return List<String> of the XAttr names of the file or directory

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FileSystem.java

@@ -2364,21 +2364,10 @@ public abstract class FileSystem extends Configured implements Closeable {
 
   /**
    * Set an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * A regular user can only set an xattr for the "user" namespace.
-   * The super user can set an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set when the logged-in user has the correct permissions.
-   * If the xattr exists, it will be replaced.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to modify
    * @param name xattr name.
@@ -2393,21 +2382,10 @@ public abstract class FileSystem extends Configured implements Closeable {
 
   /**
    * Set an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
-   * <p/>
-   * A regular user can only set an xattr for the "user" namespace.
-   * The super user can set an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set if the logged-in user has the correct permissions.
-   * If the xattr exists, it is replaced.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to modify
    * @param name xattr name.
@@ -2423,20 +2401,10 @@ public abstract class FileSystem extends Configured implements Closeable {
 
   /**
    * Get an xattr name and value for a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
-   * <p/>
-   * 
-   * A regular user can only get an xattr for the "user" namespace.
-   * The super user can get an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * An xattr will only be returned if the logged-in user has the
-   * correct permissions.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attribute
    * @param name xattr name.
@@ -2453,13 +2421,7 @@ public abstract class FileSystem extends Configured implements Closeable {
    * Only those xattrs which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattrs for the "user" namespace.
-   * The super user can only get xattrs for "user" and "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed
-   * internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @return Map<String, byte[]> describing the XAttrs of the file or directory
@@ -2475,13 +2437,7 @@ public abstract class FileSystem extends Configured implements Closeable {
    * Only those xattrs which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattrs for the "user" namespace.
-   * The super user can only get xattrs for "user" and "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed
-   * internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @param names XAttr names.
@@ -2499,14 +2455,7 @@ public abstract class FileSystem extends Configured implements Closeable {
    * Only those xattr names which the logged-in user has permissions to view
    * are returned.
    * <p/>
-   * A regular user can only get xattr names for the "user" namespace.
-   * The super user can only get xattr names for "user" and "trusted"
-   * namespaces.
-   * The xattrs of the "security" and "system" namespaces are only
-   * used/exposed internally by/to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to get extended attributes
    * @return List<String> of the XAttr names of the file or directory
@@ -2519,21 +2468,10 @@ public abstract class FileSystem extends Configured implements Closeable {
 
   /**
    * Remove an xattr of a file or directory.
-   * The name must be prefixed with user/trusted/security/system and
-   * followed by ".". For example, "user.attr".
-   * <p/>
-   * A regular user can only remove an xattr for the "user" namespace.
-   * The super user can remove an xattr of either the "user" or "trusted" namespaces.
-   * The xattrs of the "security" and "system" namespaces are only used/exposed 
-   * internally by/to the FS impl.
-   * <p/>
-   * The access permissions of an xattr in the "user" namespace are
-   * defined by the file and directory permission bits.
-   * An xattr can only be set when the logged-in user has the correct permissions.
-   * If the xattr exists, it will be replaced.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
    *
    * @param path Path to remove extended attribute
    * @param name xattr name

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -329,6 +329,9 @@ Release 2.6.0 - UNRELEASED
     HDFS-6665. Add tests for XAttrs in combination with viewfs.
     (Stephen Chu via wang)
 
+    HDFS-6778. The extended attributes javadoc should simply refer to the
+    user docs. (clamb via wang)
+
   OPTIMIZATIONS
 
     HDFS-6690. Deduplicate xattr names in memory. (wang)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/protocol/ClientProtocol.java

@@ -1267,17 +1267,11 @@ public interface ClientProtocol {
   
   /**
    * Set xattr of a file or directory.
-   * A regular user only can set xattr of "user" namespace.
-   * A super user can set xattr of "user" and "trusted" namespace.
-   * XAttr of "security" and "system" namespace is only used/exposed 
-   * internally to the FS impl.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * For xattr of "user" namespace, its access permissions are 
-   * defined by the file or directory permission bits.
-   * XAttr will be set only when login user has correct permissions.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
+   *
    * @param src file or directory
    * @param xAttr <code>XAttr</code> to set
    * @param flag set flag
@@ -1288,18 +1282,13 @@ public interface ClientProtocol {
       throws IOException;
   
   /**
-   * Get xattrs of file or directory. Values in xAttrs parameter are ignored.
-   * If xattrs is null or empty, equals getting all xattrs of the file or 
-   * directory.
-   * Only xattrs which login user has correct permissions will be returned. 
+   * Get xattrs of a file or directory. Values in xAttrs parameter are ignored.
+   * If xAttrs is null or empty, this is the same as getting all xattrs of the
+   * file or directory.  Only those xattrs for which the logged-in user has
+   * permissions to view are returned.
    * <p/>
-   * A regular user only can get xattr of "user" namespace.
-   * A super user can get xattr of "user" and "trusted" namespace.
-   * XAttr of "security" and "system" namespace is only used/exposed 
-   * internally to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
+   *
    * @param src file or directory
    * @param xAttrs xAttrs to get
    * @return List<XAttr> <code>XAttr</code> list 
@@ -1314,13 +1303,8 @@ public interface ClientProtocol {
    * Only the xattr names for which the logged in user has the permissions to
    * access will be returned.
    * <p/>
-   * A regular user only can get xattr names from the "user" namespace.
-   * A super user can get xattr names of the "user" and "trusted" namespace.
-   * XAttr names of the "security" and "system" namespaces are only used/exposed
-   * internally by the file system impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
+   *
    * @param src file or directory
    * @param xAttrs xAttrs to get
    * @return List<XAttr> <code>XAttr</code> list
@@ -1332,15 +1316,11 @@ public interface ClientProtocol {
   
   /**
    * Remove xattr of a file or directory.Value in xAttr parameter is ignored.
-   * Name must be prefixed with user/trusted/security/system.
+   * The name must be prefixed with the namespace followed by ".". For example,
+   * "user.attr".
    * <p/>
-   * A regular user only can remove xattr of "user" namespace.
-   * A super user can remove xattr of "user" and "trusted" namespace.
-   * XAttr of "security" and "system" namespace is only used/exposed 
-   * internally to the FS impl.
-   * <p/>
-   * @see <a href="http://en.wikipedia.org/wiki/Extended_file_attributes">
-   * http://en.wikipedia.org/wiki/Extended_file_attributes</a>
+   * Refer to the HDFS extended attributes user documentation for details.
+   *
    * @param src file or directory
    * @param xAttr <code>XAttr</code> to remove
    * @throws IOException