Hoist some ArchiveMount logic into a new superclass

- Add AbstractInMemoryMount, which contains all of ArchiveMount's file
   tree logic, but not the caching functionality.

 - Convert MemoryMount to inherit from AbstractInMemoryMount.

 - Add a helper method to add a file to an AbstractInMemoryMount, and
   use that within {Resource,Jar}Mount.

There's definitely more work to be done here - it might be nice to split
FileEntry into separate Directory and File interfaces, or at least make
them slightly more immutable, but that's definitely a future job.

projects/common/src/main/java/dan200/computercraft/shared/computer/core/ResourceMount.java

@@ -29,8 +29,6 @@ import java.util.Map;
 public final class ResourceMount extends ArchiveMount<ResourceMount.FileEntry> {
     private static final Logger LOG = LoggerFactory.getLogger(ResourceMount.class);
 
-    private static final byte[] TEMP_BUFFER = new byte[8192];
-
     /**
      * Maintain a cache of currently loaded resource mounts. This cache is invalidated when currentManager changes.
      */
@@ -68,7 +66,11 @@ public final class ResourceMount extends ArchiveMount<ResourceMount.FileEntry> {
             if (!FileSystem.contains(subPath, file.getPath())) continue; // Some packs seem to include the parent?
 
             var localPath = FileSystem.toLocal(file.getPath(), subPath);
-            create(newRoot, localPath);
+            try {
+                getOrCreateChild(newRoot, localPath, this::createEntry);
+            } catch (ResourceLocationException e) {
+                LOG.warn("Cannot create resource location for {} ({})", localPath, e.getMessage());
+            }
             hasAny = true;
         }
 
@@ -83,52 +85,12 @@ public final class ResourceMount extends ArchiveMount<ResourceMount.FileEntry> {
         }
     }
 
-    private void create(FileEntry lastEntry, String path) {
+    private FileEntry createEntry(String path) {
-        var lastIndex = 0;
+        return new FileEntry(path, new ResourceLocation(namespace, subPath + "/" + path));
-        while (lastIndex < path.length()) {
-            var nextIndex = path.indexOf('/', lastIndex);
-            if (nextIndex < 0) nextIndex = path.length();
-
-            var part = path.substring(lastIndex, nextIndex);
-            if (lastEntry.children == null) lastEntry.children = new HashMap<>();
-
-            var nextEntry = lastEntry.children.get(part);
-            if (nextEntry == null) {
-                ResourceLocation childPath;
-                try {
-                    childPath = new ResourceLocation(namespace, subPath + "/" + path);
-                } catch (ResourceLocationException e) {
-                    LOG.warn("Cannot create resource location for {} ({})", part, e.getMessage());
-                    return;
-                }
-                lastEntry.children.put(part, nextEntry = new FileEntry(path, childPath));
-            }
-
-            lastEntry = nextEntry;
-            lastIndex = nextIndex + 1;
-        }
     }
 
     @Override
-    public long getSize(FileEntry file) {
+    public byte[] getFileContents(FileEntry file) throws IOException {
-        var resource = manager.getResource(file.identifier).orElse(null);
-        if (resource == null) return 0;
-
-        try (var stream = resource.open()) {
-            int total = 0, read = 0;
-            do {
-                total += read;
-                read = stream.read(TEMP_BUFFER);
-            } while (read > 0);
-
-            return total;
-        } catch (IOException e) {
-            return 0;
-        }
-    }
-
-    @Override
-    public byte[] getContents(FileEntry file) throws IOException {
         var resource = manager.getResource(file.identifier).orElse(null);
         if (resource == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
 

projects/core-api/src/main/java/dan200/computercraft/api/filesystem/FileAttributes.java

@@ -12,25 +12,30 @@ import java.time.Instant;
 /**
  * A simple version of {@link BasicFileAttributes}, which provides what information a {@link Mount} already exposes.
  *
- * @param isDirectory Whether this filesystem entry is a directory.
+ * @param isDirectory      Whether this filesystem entry is a directory.
- * @param size        The size of the file.
+ * @param size             The size of the file.
+ * @param creationTime     The time the file was created.
+ * @param lastModifiedTime The time the file was last modified.
  */
-public record FileAttributes(boolean isDirectory, long size) implements BasicFileAttributes {
+public record FileAttributes(
+    boolean isDirectory, long size, FileTime creationTime, FileTime lastModifiedTime
+) implements BasicFileAttributes {
     private static final FileTime EPOCH = FileTime.from(Instant.EPOCH);
 
-    @Override
+    /**
-    public FileTime lastModifiedTime() {
+     * Create a new {@link FileAttributes} instance with the {@linkplain #creationTime() creation time} and
-        return EPOCH;
+     * {@linkplain #lastModifiedTime() last modified time} set to the Unix epoch.
+     *
+     * @param isDirectory Whether the filesystem entry is a directory.
+     * @param size        The size of the file.
+     */
+    public FileAttributes(boolean isDirectory, long size) {
+        this(isDirectory, size, EPOCH, EPOCH);
     }
 
     @Override
     public FileTime lastAccessTime() {
-        return EPOCH;
+        return lastModifiedTime();
-    }
-
-    @Override
-    public FileTime creationTime() {
-        return EPOCH;
     }
 
     @Override

projects/core/src/main/java/dan200/computercraft/core/apis/FSAPI.java

@@ -17,7 +17,6 @@ import dan200.computercraft.core.filesystem.FileSystemException;
 import dan200.computercraft.core.metrics.Metrics;
 
 import javax.annotation.Nullable;
-import java.nio.file.attribute.FileTime;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.function.Function;
@@ -488,9 +487,9 @@ public class FSAPI implements ILuaAPI {
         try (var ignored = environment.time(Metrics.FS_OPS)) {
             var attributes = getFileSystem().getAttributes(path);
             Map<String, Object> result = new HashMap<>();
-            result.put("modification", getFileTime(attributes.lastModifiedTime()));
+            result.put("modification", attributes.lastModifiedTime().toMillis());
-            result.put("modified", getFileTime(attributes.lastModifiedTime()));
+            result.put("modified", attributes.lastModifiedTime().toMillis());
-            result.put("created", getFileTime(attributes.creationTime()));
+            result.put("created", attributes.creationTime().toMillis());
             result.put("size", attributes.isDirectory() ? 0 : attributes.size());
             result.put("isDir", attributes.isDirectory());
             result.put("isReadOnly", getFileSystem().isReadOnly(path));
@@ -499,8 +498,4 @@ public class FSAPI implements ILuaAPI {
             throw new LuaException(e.getMessage());
         }
     }
-
-    private static long getFileTime(@Nullable FileTime time) {
-        return time == null ? 0 : time.toMillis();
-    }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/AbstractInMemoryMount.java

@@ -0,0 +1,152 @@
+// SPDX-FileCopyrightText: 2022 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.core.filesystem;
+
+import dan200.computercraft.api.filesystem.FileAttributes;
+import dan200.computercraft.api.filesystem.FileOperationException;
+import dan200.computercraft.api.filesystem.Mount;
+
+import javax.annotation.Nullable;
+import java.io.IOException;
+import java.nio.channels.SeekableByteChannel;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+
+/**
+ * An abstract mount which stores its file tree in memory.
+ *
+ * @param <T> The type of file.
+ */
+public abstract class AbstractInMemoryMount<T extends AbstractInMemoryMount.FileEntry<T>> implements Mount {
+    protected static final String NO_SUCH_FILE = "No such file";
+
+    @Nullable
+    protected T root;
+
+    private @Nullable T get(String path) {
+        var lastEntry = root;
+        var lastIndex = 0;
+
+        while (lastEntry != null && lastIndex < path.length()) {
+            var nextIndex = path.indexOf('/', lastIndex);
+            if (nextIndex < 0) nextIndex = path.length();
+
+            lastEntry = lastEntry.children == null ? null : lastEntry.children.get(path.substring(lastIndex, nextIndex));
+            lastIndex = nextIndex + 1;
+        }
+
+        return lastEntry;
+    }
+
+    @Override
+    public final boolean exists(String path) {
+        return get(path) != null;
+    }
+
+    @Override
+    public final boolean isDirectory(String path) {
+        var file = get(path);
+        return file != null && file.isDirectory();
+    }
+
+    @Override
+    public final void list(String path, List<String> contents) throws IOException {
+        var file = get(path);
+        if (file == null || !file.isDirectory()) throw new FileOperationException(path, "Not a directory");
+
+        file.list(contents);
+    }
+
+    @Override
+    public final long getSize(String path) throws IOException {
+        var file = get(path);
+        if (file == null) throw new FileOperationException(path, NO_SUCH_FILE);
+        return getSize(file);
+    }
+
+    /**
+     * Get the size of a file.
+     *
+     * @param file The file to get the size of.
+     * @return The size of the file. This should be 0 for directories, and equal to {@code openForRead(_).size()} for files.
+     * @throws IOException If the size could not be read.
+     */
+    protected abstract long getSize(T file) throws IOException;
+
+    @Override
+    public final SeekableByteChannel openForRead(String path) throws IOException {
+        var file = get(path);
+        if (file == null || file.isDirectory()) throw new FileOperationException(path, NO_SUCH_FILE);
+        return openForRead(file);
+    }
+
+    /**
+     * Open a file for reading.
+     *
+     * @param file The file to read. This will not be a directory.
+     * @return The channel for this file.
+     */
+    protected abstract SeekableByteChannel openForRead(T file) throws IOException;
+
+    @Override
+    public final BasicFileAttributes getAttributes(String path) throws IOException {
+        var file = get(path);
+        if (file == null) throw new FileOperationException(path, NO_SUCH_FILE);
+        return getAttributes(file);
+    }
+
+    /**
+     * Get all attributes of the file.
+     *
+     * @param file The file to compute attributes for.
+     * @return The file's attributes.
+     * @throws IOException If the attributes could not be read.
+     */
+    protected BasicFileAttributes getAttributes(T file) throws IOException {
+        return new FileAttributes(file.isDirectory(), getSize(file));
+    }
+
+    protected T getOrCreateChild(T lastEntry, String localPath, Function<String, T> factory) {
+        var lastIndex = 0;
+        while (lastIndex < localPath.length()) {
+            var nextIndex = localPath.indexOf('/', lastIndex);
+            if (nextIndex < 0) nextIndex = localPath.length();
+
+            var part = localPath.substring(lastIndex, nextIndex);
+            if (lastEntry.children == null) lastEntry.children = new HashMap<>(0);
+
+            var nextEntry = lastEntry.children.get(part);
+            if (nextEntry == null || !nextEntry.isDirectory()) {
+                lastEntry.children.put(part, nextEntry = factory.apply(localPath.substring(0, nextIndex)));
+            }
+
+            lastEntry = nextEntry;
+            lastIndex = nextIndex + 1;
+        }
+
+        return lastEntry;
+    }
+
+    protected static class FileEntry<T extends FileEntry<T>> {
+        public final String path;
+        @Nullable
+        public Map<String, T> children;
+
+        protected FileEntry(String path) {
+            this.path = path;
+        }
+
+        public boolean isDirectory() {
+            return children != null;
+        }
+
+        protected void list(List<String> contents) {
+            if (children != null) contents.addAll(children.keySet());
+        }
+    }
+}

projects/core/src/main/java/dan200/computercraft/core/filesystem/ArchiveMount.java

@@ -6,25 +6,21 @@ package dan200.computercraft.core.filesystem;
 
 import com.google.common.cache.Cache;
 import com.google.common.cache.CacheBuilder;
-import dan200.computercraft.api.filesystem.FileAttributes;
-import dan200.computercraft.api.filesystem.FileOperationException;
-import dan200.computercraft.api.filesystem.Mount;
 import dan200.computercraft.core.apis.handles.ArrayByteChannel;
 
-import javax.annotation.Nullable;
 import java.io.IOException;
 import java.nio.channels.SeekableByteChannel;
-import java.nio.file.attribute.BasicFileAttributes;
-import java.util.List;
-import java.util.Map;
 import java.util.concurrent.TimeUnit;
 
 /**
  * An abstract mount based on some archive of files, such as a Zip or Minecraft's resources.
+ * <p>
+ * We assume that we cannot create {@link SeekableByteChannel}s directly from the archive, and so maintain a (shared)
+ * cache of recently read files and their contents.
  *
  * @param <T> The type of file.
  */
-public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> implements Mount {
+public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> extends AbstractInMemoryMount<T> {
     protected static final String NO_SUCH_FILE = "No such file";
 
     /**
@@ -44,82 +40,38 @@ public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> implemen
         .<FileEntry<?>, byte[]>weigher((k, v) -> v.length)
         .build();
 
-    @Nullable
-    protected T root;
-
-    private @Nullable T get(String path) {
-        var lastEntry = root;
-        var lastIndex = 0;
-
-        while (lastEntry != null && lastIndex < path.length()) {
-            var nextIndex = path.indexOf('/', lastIndex);
-            if (nextIndex < 0) nextIndex = path.length();
-
-            lastEntry = lastEntry.children == null ? null : lastEntry.children.get(path.substring(lastIndex, nextIndex));
-            lastIndex = nextIndex + 1;
-        }
-
-        return lastEntry;
-    }
-
     @Override
-    public final boolean exists(String path) {
+    protected final long getSize(T file) throws IOException {
-        return get(path) != null;
-    }
-
-    @Override
-    public final boolean isDirectory(String path) {
-        var file = get(path);
-        return file != null && file.isDirectory();
-    }
-
-    @Override
-    public final void list(String path, List<String> contents) throws IOException {
-        var file = get(path);
-        if (file == null || !file.isDirectory()) throw new FileOperationException(path, "Not a directory");
-
-        file.list(contents);
-    }
-
-    @Override
-    public final long getSize(String path) throws IOException {
-        var file = get(path);
-        if (file == null) throw new FileOperationException(path, NO_SUCH_FILE);
-        return getCachedSize(file);
-    }
-
-    private long getCachedSize(T file) throws IOException {
         if (file.size != -1) return file.size;
         if (file.isDirectory()) return file.size = 0;
 
         var contents = CONTENTS_CACHE.getIfPresent(file);
-        if (contents != null) return file.size = contents.length;
+        return file.size = contents != null ? contents.length : getFileSize(file);
-
-        return file.size = getSize(file);
     }
 
     /**
-     * Get the size of a file.
+     * Get the size of the file by reading it (or its metadata) from disk.
-     * <p>
-     * This should only be called once per file, as the result is cached in {@link #getSize(String)}.
      *
-     * @param file The file to compute the size of. This will not be a directory.
+     * @param file The file to get the size of.
-     * @return The size of the file.
+     * @return The file's size.
-     * @throws IOException If the size could not be read.
+     * @throws IOException If the size could not be computed.
      */
-    protected abstract long getSize(T file) throws IOException;
+    protected long getFileSize(T file) throws IOException {
+        return getContents(file).length;
+    }
 
     @Override
-    public SeekableByteChannel openForRead(String path) throws IOException {
+    protected final SeekableByteChannel openForRead(T file) throws IOException {
-        var file = get(path);
+        return new ArrayByteChannel(getContents(file));
-        if (file == null || file.isDirectory()) throw new FileOperationException(path, NO_SUCH_FILE);
+    }
 
+    private byte[] getContents(T file) throws IOException {
         var cachedContents = CONTENTS_CACHE.getIfPresent(file);
-        if (cachedContents != null) return new ArrayByteChannel(cachedContents);
+        if (cachedContents != null) return cachedContents;
 
-        var contents = getContents(file);
+        var contents = getFileContents(file);
         CONTENTS_CACHE.put(file, contents);
-        return new ArrayByteChannel(contents);
+        return contents;
     }
 
     /**
@@ -128,44 +80,13 @@ public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> implemen
      * @param file The file to read into memory. This will not be a directory.
      * @return The contents of the file.
      */
-    protected abstract byte[] getContents(T file) throws IOException;
+    protected abstract byte[] getFileContents(T file) throws IOException;
-
-    @Override
-    public final BasicFileAttributes getAttributes(String path) throws IOException {
-        var file = get(path);
-        if (file == null) throw new FileOperationException(path, NO_SUCH_FILE);
-
-        return getAttributes(file);
-    }
-
-    /**
-     * Get all attributes of the file.
-     *
-     * @param file The file to compute attributes for. This will not be a directory.
-     * @return The file's attributes.
-     * @throws IOException If the attributes could not be read.
-     */
-    protected BasicFileAttributes getAttributes(T file) throws IOException {
-        return new FileAttributes(file.isDirectory(), getCachedSize(file));
-    }
-
-    protected static class FileEntry<T extends ArchiveMount.FileEntry<T>> {
-        public final String path;
-        @Nullable
-        public Map<String, T> children;
 
+    protected static class FileEntry<T extends FileEntry<T>> extends AbstractInMemoryMount.FileEntry<T> {
         long size = -1;
 
         protected FileEntry(String path) {
-            this.path = path;
+            super(path);
-        }
-
-        protected boolean isDirectory() {
-            return children != null;
-        }
-
-        protected void list(List<String> contents) {
-            if (children != null) contents.addAll(children.keySet());
         }
     }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/JarMount.java

@@ -4,8 +4,8 @@
 
 package dan200.computercraft.core.filesystem;
 
+import dan200.computercraft.api.filesystem.FileAttributes;
 import dan200.computercraft.api.filesystem.FileOperationException;
-import dan200.computercraft.core.util.Nullability;
 
 import javax.annotation.Nullable;
 import java.io.Closeable;
@@ -42,7 +42,7 @@ public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closea
         }
 
         // Read in all the entries
-        root = new FileEntry("");
+        var root = this.root = new FileEntry("");
         var zipEntries = zip.entries();
         while (zipEntries.hasMoreElements()) {
             var entry = zipEntries.nextElement();
@@ -51,41 +51,18 @@ public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closea
             if (!entryPath.startsWith(subPath)) continue;
 
             var localPath = FileSystem.toLocal(entryPath, subPath);
-            create(entry, localPath);
+            getOrCreateChild(root, localPath, FileEntry::new).setup(entry);
         }
     }
 
-    private void create(ZipEntry entry, String localPath) {
-        var lastEntry = Nullability.assertNonNull(root);
-
-        var lastIndex = 0;
-        while (lastIndex < localPath.length()) {
-            var nextIndex = localPath.indexOf('/', lastIndex);
-            if (nextIndex < 0) nextIndex = localPath.length();
-
-            var part = localPath.substring(lastIndex, nextIndex);
-            if (lastEntry.children == null) lastEntry.children = new HashMap<>(0);
-
-            var nextEntry = lastEntry.children.get(part);
-            if (nextEntry == null || !nextEntry.isDirectory()) {
-                lastEntry.children.put(part, nextEntry = new FileEntry(localPath.substring(0, nextIndex)));
-            }
-
-            lastEntry = nextEntry;
-            lastIndex = nextIndex + 1;
-        }
-
-        lastEntry.setup(entry);
-    }
-
     @Override
-    protected long getSize(FileEntry file) throws FileOperationException {
+    protected long getFileSize(FileEntry file) throws FileOperationException {
         if (file.zipEntry == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
         return file.zipEntry.getSize();
     }
 
     @Override
-    protected byte[] getContents(FileEntry file) throws FileOperationException {
+    protected byte[] getFileContents(FileEntry file) throws FileOperationException {
         if (file.zipEntry == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
 
         try (var stream = zip.getInputStream(file.zipEntry)) {
@@ -97,9 +74,10 @@ public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closea
     }
 
     @Override
-    public BasicFileAttributes getAttributes(FileEntry file) throws FileOperationException {
+    protected BasicFileAttributes getAttributes(FileEntry file) throws IOException {
-        if (file.zipEntry == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
+        return file.zipEntry == null ? super.getAttributes(file) : new FileAttributes(
-        return new ZipEntryAttributes(file.zipEntry);
+            file.isDirectory(), getSize(file), orEpoch(file.zipEntry.getCreationTime()), orEpoch(file.zipEntry.getLastModifiedTime())
+        );
     }
 
     @Override
@@ -117,69 +95,13 @@ public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closea
 
         void setup(ZipEntry entry) {
             zipEntry = entry;
-            size = entry.getSize();
             if (children == null && entry.isDirectory()) children = new HashMap<>(0);
         }
     }
 
-    private static class ZipEntryAttributes implements BasicFileAttributes {
+    private static final FileTime EPOCH = FileTime.from(Instant.EPOCH);
-        private final ZipEntry entry;
 
-        ZipEntryAttributes(ZipEntry entry) {
+    private static FileTime orEpoch(@Nullable FileTime time) {
-            this.entry = entry;
+        return time == null ? EPOCH : time;
-        }
-
-        @Override
-        public FileTime lastModifiedTime() {
-            return orEpoch(entry.getLastModifiedTime());
-        }
-
-        @Override
-        public FileTime lastAccessTime() {
-            return orEpoch(entry.getLastAccessTime());
-        }
-
-        @Override
-        public FileTime creationTime() {
-            var time = entry.getCreationTime();
-            return time == null ? lastModifiedTime() : time;
-        }
-
-        @Override
-        public boolean isRegularFile() {
-            return !entry.isDirectory();
-        }
-
-        @Override
-        public boolean isDirectory() {
-            return entry.isDirectory();
-        }
-
-        @Override
-        public boolean isSymbolicLink() {
-            return false;
-        }
-
-        @Override
-        public boolean isOther() {
-            return false;
-        }
-
-        @Override
-        public long size() {
-            return entry.getSize();
-        }
-
-        @Nullable
-        @Override
-        public Object fileKey() {
-            return null;
-        }
-
-        private static final FileTime EPOCH = FileTime.from(Instant.EPOCH);
-
-        private static FileTime orEpoch(@Nullable FileTime time) {
-            return time == null ? EPOCH : time;
-        }
     }
 }

projects/core/src/testFixtures/java/dan200/computercraft/test/core/filesystem/MemoryMount.java

@@ -7,53 +7,44 @@ package dan200.computercraft.test.core.filesystem;
 import dan200.computercraft.api.filesystem.FileOperationException;
 import dan200.computercraft.api.filesystem.Mount;
 import dan200.computercraft.core.apis.handles.ArrayByteChannel;
+import dan200.computercraft.core.filesystem.AbstractInMemoryMount;
+import dan200.computercraft.core.util.Nullability;
 
+import javax.annotation.Nullable;
+import java.io.IOException;
 import java.nio.channels.SeekableByteChannel;
 import java.nio.charset.StandardCharsets;
-import java.util.*;
 
 /**
  * A read-only mount {@link Mount} which provides a list of in-memory set of files.
  */
-public class MemoryMount implements Mount {
+public class MemoryMount extends AbstractInMemoryMount<MemoryMount.FileEntry> {
-    private final Map<String, byte[]> files = new HashMap<>();
-    private final Set<String> directories = new HashSet<>();
-
     public MemoryMount() {
-        directories.add("");
+        root = new FileEntry("");
-    }
-
-    @Override
-    public boolean exists(String path) {
-        return files.containsKey(path) || directories.contains(path);
-    }
-
-    @Override
-    public boolean isDirectory(String path) {
-        return directories.contains(path);
-    }
-
-    @Override
-    public void list(String path, List<String> files) {
-        for (var file : this.files.keySet()) {
-            if (file.startsWith(path)) files.add(file.substring(path.length() + 1));
-        }
-    }
-
-    @Override
-    public long getSize(String path) {
-        throw new RuntimeException("Not implemented");
-    }
-
-    @Override
-    public SeekableByteChannel openForRead(String path) throws FileOperationException {
-        var file = files.get(path);
-        if (file == null) throw new FileOperationException(path, "File not found");
-        return new ArrayByteChannel(file);
     }
 
     public MemoryMount addFile(String file, String contents) {
-        files.put(file, contents.getBytes(StandardCharsets.UTF_8));
+        getOrCreateChild(Nullability.assertNonNull(root), file, FileEntry::new).contents = contents.getBytes(StandardCharsets.UTF_8);
         return this;
     }
+
+    @Override
+    protected long getSize(FileEntry file) {
+        return file.contents == null ? 0 : file.contents.length;
+    }
+
+    @Override
+    protected SeekableByteChannel openForRead(FileEntry file) throws IOException {
+        if (file.contents == null) throw new FileOperationException(file.path, "File is a directory");
+        return new ArrayByteChannel(file.contents);
+    }
+
+    protected static class FileEntry extends AbstractInMemoryMount.FileEntry<FileEntry> {
+        @Nullable
+        byte[] contents;
+
+        protected FileEntry(String path) {
+            super(path);
+        }
+    }
 }