FileHandle: infer mode based on file permissions

Instead of always trying read/write, which throws an exception
for existing but read-only files.

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

@@ -29,8 +29,11 @@
 
 package org.scijava.io.handle;
 
+import java.io.File;
 import java.io.IOException;
 import java.io.RandomAccessFile;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.Date;
 
 import org.scijava.io.location.FileLocation;
@@ -50,7 +53,7 @@ public class FileHandle extends AbstractDataHandle<FileLocation> {
  private RandomAccessFile raf;
 
  /** The mode of the {@link RandomAccessFile}. */
- private String mode = "rw";
+ private String mode;
 
  /** True iff the {@link #close()} has already been called. */
  private boolean closed;
@@ -232,6 +235,28 @@ public synchronized void close() throws IOException {
  closed = true;
  }
 
+ // -- WrapperPlugin methods --
+
+ @Override
+ public void set(FileLocation loc) {
+ super.set(loc);
+
+ // Infer the initial mode based on file existence + permissions.
+ final File file = loc.getFile();
+ String mode;
+ if (file.exists()) {
+ final Path path = loc.getFile().toPath();
+ mode = "";
+ if (Files.isReadable(path)) mode += "r";
+ if (Files.isWritable(path)) mode += "w";
+ }
+ else {
+ // Non-existent file; assume the intent is to create it.
+ mode = "rw";
+ }
+ setMode(mode);
+ }
+
  // -- Typed methods --
 
  @Override