Add support for read-only and write-only handles

Signed-off-by: Curtis Rueden <ctrueden@wisc.edu>

Author: Gabriel Einsdorf

src/main/java/org/scijava/io/BytesHandle.java

@@ -116,6 +116,23 @@ public void seek(final long pos) throws IOException {
 		bytes().position((int) pos);
 	}
 
+	@Override
+	public boolean isReadable() {
+		return true;
+	}
+
+	@Override
+	public boolean isWritable() {
+		// NB: Some ByteBuffers are read-only. But there is no API to check it.
+		// Therefore, we make a "best effort" guess based on known read-only types.
+		// Since these read-only types are not public, we compare class names rather
+		// than checking for type equality or using instanceof.
+		final String className = getClass().getName();
+		final boolean readOnly = className.equals("java.nio.HeapByteBufferR") ||
+			className.equals("java.nio.DirectByteBufferR");
+		return !readOnly;
+	}
+
 	// -- DataInput methods --
 
 	@Override

src/main/java/org/scijava/io/DataHandle.java

@@ -115,15 +115,23 @@ default long available(final long count) throws IOException {
 		return remain < count ? remain : count;
 	}
 
+	/** Gets whether reading from this handle is supported. */
+	boolean isReadable();
+
+	/** Gets whether writing to this handle is supported. */
+	boolean isWritable();
+
 	/**
 	 * Ensures that the handle has sufficient bytes available to read.
 	 * 
 	 * @param count Number of bytes to read.
 	 * @see #available(long)
 	 * @throws EOFException If there are insufficient bytes available.
-	 * @throws IOException If something goes wrong with the check.
+	 * @throws IOException If the handle is write-only, or something goes wrong
+	 *           with the check.
 	 */
 	default void ensureReadable(final long count) throws IOException {
+		if (!isReadable()) throw new IOException("This handle is write-only.");
 		if (available(count) < count) throw new EOFException();
 	}
 
@@ -134,10 +142,12 @@ default void ensureReadable(final long count) throws IOException {
 	 * @param count Number of bytes to write.
 	 * @return {@code true} if the handle's length was sufficient, or
 	 *         {@code false} if the handle's length required an extension.
-	 * @throws IOException If something goes wrong with the check, or there is an
-	 *           error changing the handle's length.
+	 * @throws IOException If the handle is read-only, or something goes wrong
+	 *           with the check, or there is an error changing the handle's
+	 *           length.
 	 */
 	default boolean ensureWritable(final long count) throws IOException {
+		if (!isWritable()) throw new IOException("This handle is read-only.");
 		final long minLength = offset() + count;
 		if (length() < minLength) {
 			setLength(minLength);

src/main/java/org/scijava/io/FileHandle.java

@@ -87,6 +87,16 @@ public void setLength(final long length) throws IOException {
 		raf().setLength(length);
 	}
 
+	@Override
+	public boolean isReadable() {
+		return getMode().contains("r");
+	}
+
+	@Override
+	public boolean isWritable() {
+		return getMode().contains("w");
+	}
+
 	@Override
 	public int read() throws IOException {
 		return raf().read();