Compute blob hash in trigger on blob's table

My environment is Oracle 11g. I have a table T_IMG whose columns are an image blob, an image identifier, and the image's hash computed as SHA256. I want the image hash to be computed and inserted into the T_IMG table every time a row is inserted or updated in the T_IMG table. For this I use a before trigger on insert or update on the T_IMG table but I have difficulty accessing the image blob column inside the trigger. ``` CREATE TABLE TEST.T_IMG ( IDN VARCHAR2(18 BYTE) NOT NULL, IMG BLOB NOT NULL, IMGHASH VARCHAR2(128 BYTE) ); ``` The SHA256 hash for strings and blobs is computed by the following code that is based on Sean Stuber's example at http://seanstuber.wordpress.com/2012/03/22/using-java-to-extend-dbms_crypto/ ``` CREATE OR REPLACE AND RESOLVE JAVA SOURCE NAMED sha2 AS import java.security.MessageDigest; import oracle.sql.*; public class sha2 { public static oracle.sql.RAW get_digest_string( String p_string, int p_bits ) throws Exception { MessageDigest v_md = MessageDigest.getInstance( "SHA-" + p_bits ); byte[] v_digest; v_digest = v_md.digest( p_string.getBytes( "UTF-8" ) ); return RAW.newRAW(v_digest); } public static oracle.sql.RAW get_digest_blob( oracle.sql.BLOB p_blob, int p_bits ) throws Exception { byte[] allBytesInBlob; allBytesInBlob = p_blob.getBytes(1, (int) p_blob.length()); MessageDigest v_md = MessageDigest.getInstance( "SHA-" + p_bits ); byte[] v_digest = v_md.digest( allBytesInBlob ); return RAW.newRAW(v_digest); } } / CREATE OR REPLACE FUNCTION sha2_string(p_string in VARCHAR2, p_bits in number) RETURN RAW AS LANGUAGE JAVA NAME 'sha2.get_digest_string( java.lang.String, int ) return oracle.sql.RAW'; / CREATE OR REPLACE FUNCTION sha2_blob(p_byte in BLOB, p_bits in number) RETURN RAW AS LANGUAGE JAVA NAME 'sha2.get_digest_blob( oracle.sql.BLOB, int ) return oracle.sql.RAW'; / ``` The above functions work. Testing them I get: ``` SELECT sha2_string('0123456789',256) FROM DUAL; SELECT sha2_blob(utl_raw.cast_to_raw('0123456789'),256) FROM DUAL; ``` SHA2_STRING('0123456789',256) -------------------------------------------------------------------------------- 84D89877F0D4041EFB6BF91A16F0248F2FD573E6AF05C19F96BEDB9F882F7882 1 row selected. SHA2_BLOB(UTL_RAW.CAST_TO_RAW('0123456789'),256) -------------------------------------------------------------------------------- 84D89877F0D4041EFB6BF91A16F0248F2FD573E6AF05C19F96BEDB9F882F7882 1 row selected. I use a before trigger on insert or update on T_IMG to compute the SH256 hashing of the image column. ``` CREATE OR REPLACE TRIGGER TEST.TR_IMG_UPSERT BEFORE INSERT OR UPDATE ON TEST.T_IMG FOR EACH ROW DECLARE imghash varchar2(128); vblob blob; BEGIN vblob := :new.IMG; IF(vblob IS NOT NULL)THEN /*-- this works imghash := sha2_string('0123456789',256); :new.imghash := imghash; */ /*-- this works too vblob := utl_raw.cast_to_raw('0123456789'); imghash := sha2_blob(vblob,256); :new.imghash := imghash; */ -- this DOES NOT work imghash := sha2_blob(vblob,256); :new.imghash := imghash; END IF; END; / ``` The trigger by itself works; where it computes and inserts the hash value on mock-up data (manufactured string or blob) but not on the real image blob data when it throws the following error: ORA-29532: Java call terminated by uncaught Java exception: java.sql.SQLException: Invalid empty lob operation ORA-06512: at "TEST.SHA2_BLOB", line 1 ORA-06512: at "TEST.TR_IMG_UPSERT", line 13 ORA-04088: error during execution of trigger 'TEST.TR_IMG_UPSERT' It seems that the trigger reads the :new.IMG data as an empty blob. Why this happens and how can I solve my problem of automatically computing and inserting the image's hash into the T_IMG table every time a row is inserted or updated in the T_IMG table? Note: The hash can not be provided by the client app inserting/updating the data rows. AdiM