This CL adds missing Instant-based methods to FileTime in J2ObjC JRE emulation to satisfy symbol resolution during transpilation of apache_commons_io.

jre_emul/Classes/java/nio/file/attribute/FileTime.java

@@ -17,6 +17,7 @@
 
 package java.nio.file.attribute;
 
+import java.time.Instant;
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -52,6 +53,40 @@ public static FileTime fromMillis(long value) {
     return new FileTime(value);
   }
 
+  public native Instant toInstant() /*-[
+    id p = [JavaNioFileAttributeFileTime getProvider];
+    if (p) {
+      return [p toInstantWithJavaNioFileAttributeFileTime:self];
+    }
+    @throw create_JavaLangUnsupportedOperationException_initWithNSString_(@"java.time is not available");
+  ]-*/;
+
+  public static FileTime from(Instant instant) {
+    return getProvider().fromInstant(instant);
+  }
+
+  private static Provider provider;
+
+  private static Provider getProvider() {
+    if (provider == null) {
+      try {
+        Class.forName("java.nio.file.attribute.FileTimeProvider").newInstance();
+      } catch (Exception e) {
+        throw new UnsupportedOperationException("java.time is not available", e);
+      }
+    }
+    return provider;
+  }
+
+  public interface Provider {
+    Object toInstant(FileTime fileTime);
+    FileTime fromInstant(Object instant);
+  }
+
+  public static void setProvider(Provider p) {
+    provider = p;
+  }
+
   @Override
   public int compareTo(FileTime o) {
     if (o.millis < millis) {

jre_emul/Classes/java/nio/file/attribute/FileTimeProvider.java

@@ -0,0 +1,19 @@
+package java.nio.file.attribute;
+
+import java.time.Instant;
+
+public class FileTimeProvider implements FileTime.Provider {
+  public FileTimeProvider() {
+    FileTime.setProvider(this);
+  }
+
+  @Override
+  public Object toInstant(FileTime fileTime) {
+    return Instant.ofEpochMilli(fileTime.toMillis());
+  }
+
+  @Override
+  public FileTime fromInstant(Object instant) {
+    return FileTime.fromMillis(((Instant) instant).toEpochMilli());
+  }
+}