Hfrhyu

Although I got a hint from Ian, I still have a problem to read a GeoTIFF in Geotools.
I ask this as a new question so that I can put all info.

The GeoTIFF is from
http://cidportal.jrc.ec.europa.eu/ftp/jrc-opendata/GHSL/GHS_POP_GPW4_GLOBE_R2015A/GHS_POP_GPW42015_GLOBE_R2015A_54009_1k/V1-0/

gdalinfo GHS_POP_GPW42015_GLOBE_R2015A_54009_1k_v1_0.tif
Driver: GTiff/GeoTIFF
Files: GHS_POP_GPW42015_GLOBE_R2015A_54009_1k_v1_0.tif
 GHS_POP_GPW42015_GLOBE_R2015A_54009_1k_v1_0.tif.ovr
 GHS_POP_GPW42015_GLOBE_R2015A_54009_1k_v1_0.tif.aux.xml
Size is 35497, 15236
Coordinate System is:
PROJCS["World_Mollweide",
 GEOGCS["GCS_WGS_1984",
 DATUM["D_WGS_1984",
 SPHEROID["WGS_1984",6378137.0,298.257223563]],
 PRIMEM["Greenwich",0.0],
 UNIT["Degree",0.017453292519943295]],
 PROJECTION["Mollweide"],
 PARAMETER["False_Easting",0.0],
 PARAMETER["False_Northing",0.0],
 PARAMETER["Central_Meridian",0.0],
 UNIT["Meter",1.0]]

As it is a Mollweide projection, I add the EPGS definition.

epsg.properties file:

54009=PROJCS["World_Mollweide",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Mollweide"],PARAMETER["Central_Meridian",0.0],UNIT["Meter",1.0]]

 URL epsg = getClass().getClassLoader().getResource("epsg.properties");

 Hints hints = new Hints(Hints.CRS_AUTHORITY_FACTORY, PropertyAuthorityFactory.class);
 ReferencingFactoryContainer referencingFactoryContainer = ReferencingFactoryContainer.instance(hints);
 PropertyAuthorityFactory factory;
 factory = new PropertyAuthorityFactory(referencingFactoryContainer, Citations.fromName("EPSG"), epsg);
 ReferencingFactoryFinder.addAuthorityFactory(factory);
 ReferencingFactoryFinder.scanForPlugins();

This prints correctly:

System.out.println(CRS.decode("EPSG:54009"));

PROJCS["World_Mollweide", 
 GEOGCS["GCS_WGS_1984", 
 DATUM["D_WGS_1984", 
 SPHEROID["WGS_1984", 6378137.0, 298.257223563]], 
 PRIMEM["Greenwich", 0.0], 
 UNIT["degree", 0.017453292519943295], 
 AXIS["Longitude", EAST], 
 AXIS["Latitude", NORTH]], 
 PROJECTION["Mollweide"], 
 PARAMETER["semi_minor", 6378137.0], 
 PARAMETER["false_easting", 0.0], 
 PARAMETER["false_northing", 0.0], 
 PARAMETER["central_meridian", 0.0], 
 UNIT["m", 1.0], 
 AXIS["x", EAST], 
 AXIS["y", NORTH], 
 AUTHORITY["EPSG","54009"]]

But

String fileName = "/home/XXX/GHS_POP_GPW42015_GLOBE_R2015A_54009_1k_v1_0.tif";

new GeoTiffReader(new FileInputStream(new File(fileName)));

still throws an error

GeoTiffMetadata2CRSAdapter::createCoordinateSystem:Only Geographic & Projected Systems are supported. 
 at org.geotools.coverage.grid.io.imageio.geotiff.GeoTiffMetadata2CRSAdapter.createCoordinateSystem(GeoTiffMetadata2CRSAdapter.java:186)
 at org.geotools.gce.geotiff.GeoTiffReader.getHRInfo(GeoTiffReader.java:329)
 ... 4 more

geotools version 21.0
maven

<dependency>
 <groupId>org.geotools</groupId>
 <artifactId>gt-main</artifactId>
 <version>$geotools.version</version>
 </dependency>

 <dependency>
 <groupId>org.geotools</groupId>
 <artifactId>gt-referencing</artifactId>
 <version>$geotools.version</version>
 </dependency>

 <dependency>
 <groupId>org.geotools</groupId>
 <artifactId>gt-epsg-wkt</artifactId>
 <version>$geotools.version</version>
 </dependency>

 <dependency>
 <groupId>org.geotools</groupId>
 <artifactId>gt-geotiff</artifactId>
 <version>$geotools.version</version>
 </dependency>

Solved !

1) I added a .prj file with the EPGS definition. It is in the same folder as the tiff file and has the same name.

2) I changed

new GeoTiffReader(new FileInputStream(new File(fileName)));

to

new GeoTiffReader(new File(fileName));

so that the .prj file is picked up.