This RSA Knowledge Base Article explains how to enable DBMS_OUTPUT in Oracle SQL Developer.
For example, you have the following PL/SQL script that writes to DBMS_OUTPUT.
DECLARE
report CLOB;
begin
report := SRSection_System.Database('Database Summary');
DBMS_OUTPUT.PUT_LINE(report);
report := SRSection_System.Product('Product Version');
DBMS_OUTPUT.PUT_LINE(report);
end;
But when you run the script, the DBMS_OUTPUT is not displayed. The following script is executed as AVUSER.
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This RSA Knowledge Base Article explains how to enable DBMS_OUTPUT to get the query results to display.