Allow C library to be loaded after Nc4Iosp initialization for some cases

If the netCDF-C library was not found on the system library path upon
initialization of the Nc4Iosp class, retry loading when calling
Nc4Iosp.setLibraryAndPath.

Clarify the javadocs for the setLibraryNameAndPath methods of the
Nc4Iosp and NetcdfClibrary classes.

Addresses #1469

Author: lesserwhirls

netcdf4/src/main/java/ucar/nc2/ffi/netcdf/NetcdfClibrary.java

@@ -32,14 +32,23 @@ public class NetcdfClibrary {
 
   /**
    * Set the path and name of the netcdf c library.
-   * Must be called before load() is called.
+   * <p>
+   * Must be called prior to calling {@link #isLibraryPresent() isLibraryPresent}
+   * or {@link #getForeignFunctionInterface() getForeignFunctionInterface}, as
+   * the C library can only be successfully loaded once.
    *
    * @param jna_path path to shared libraries, may be null. If null, will look for system property
    *        "jna.library.path", then environment variable "JNA_PATH". If set, will set
    *        the environment variable "JNA_PATH".
    * @param lib_name library name, may be null. If null, will use "netcdf".
    */
   public static void setLibraryNameAndPath(@Nullable String jna_path, @Nullable String lib_name) {
+
+    if (nc4 != null) {
+      log.warn("netCDF-C library already set, ignoring.");
+      return;
+    }
+
     lib_name = Strings.emptyToNull(lib_name);
 
     if (lib_name == null) {
@@ -61,6 +70,11 @@ public static void setLibraryNameAndPath(@Nullable String jna_path, @Nullable St
 
     libName = lib_name;
     jnaPath = jna_path;
+
+    if ((isClibraryPresent == null || !isClibraryPresent) && jnaPath != null) {
+      // call load to retry loading, but this time with jnaPath set
+      load();
+    }
   }
 
   /**

netcdf4/src/main/java/ucar/nc2/jni/netcdf/Nc4Iosp.java

@@ -74,16 +74,26 @@ public class Nc4Iosp extends AbstractIOServiceProvider implements IOServiceProvi
   private static final boolean transcodeStrings = Charset.defaultCharset() != StandardCharsets.UTF_8;
 
   /**
-   * set the path and name of the netcdf c library.
-   * must be called before load() is called.
+   * Set the path and name of the netcdf c library.
+   * <p>
+   * This method will only work if the netCDF-C library was not found on the
+   * system path. If you need to use a specific version of the C library, and
+   * you have a different version on a system path, do not use this method.
+   * Instead, call {@link NetcdfClibrary#setLibraryNameAndPath(String, String)
+   * NetcdfClibrary.setLibraryNameAndPath} prior to any uses of
+   * the Nc4Iosp class.
    *
    * @param jnaPath path to shared libraries
    * @param libName library name
    * @deprecated use NetcdfClibrary.setLibraryNameAndPath
    */
   @Deprecated
   public static void setLibraryAndPath(String jnaPath, String libName) {
-    NetcdfClibrary.setLibraryNameAndPath(jnaPath, libName);
+    if (nc4 != null) {
+      log.warn("Library already set, ignoring.");
+    } else {
+      NetcdfClibrary.setLibraryNameAndPath(jnaPath, libName);
+    }
   }
 
   /**