Analysis Details

Do Not Attempt to Help the Garbage Collector by Setting Local Reference Variables to Null

es.uniovi.reflection.analyses.cmu.obj54

This rule detects the assignment of null to local variables or parameters when they are no longer used.

Computational Reflection Research Group

01-05-2026 13:28

Programmer Misconceptions

Assignment

MATCH (varDec:LOCAL_DEC)-[:MODIFIED_BY]->(ass:ASSIGNMENT)-[:ASSIGNMENT_RHS]->(:LITERAL{typeKind:'NULL'})
WITH distinct varDec, es.uniovi.reflection.codeanalysis.functions.getEnclStmt(ass) as assStat
OPTIONAL MATCH (varDec)-[:USED_BY| STATE_MODIFIED_BY]->(useExpr:EXPRESSION)
WHERE NOT useExpr:CALL
WITH varDec, assStat, COLLECT(distinct es.uniovi.reflection.codeanalysis.functions.getEnclStmt(useExpr)) as useStats
CALL apoc.when( size(useStats) > 0,
'CALL apoc.path.subgraphNodes($assStat, {terminatorNodes:$useStats, denylistNodes:[$varDec],limit:1, minLevel:0,optional:true, relationshipFilter:"CFG_NEXT>|CFG_TRUE_CONDITION>|CFG_FALSE_CONDITION>|CFG_FOR_EACH_HAS_NEXT>|CFG_FOR_EACH_NO_NEXT>|CFG_IF_NO_EXCEPTION>|CFG_IF_CAUGHT_EXCEPTION>|CFG_IF_PREVIOUS_BREAK>|CFG_IF_PREVIOUS_CONTINUE>|CFG_CASE_MATCHES>|CFG_CASE_NO_MATCH>|CFG_IF_UNCAUGHT_EXCEPTION>|CFG_MAY_THROW>|CFG_THROWS>|TRY_CATCH>"}) YIELD node RETURN node', 'RETURN null as node',     {assStat: assStat, useStats: useStats, varDec:varDec}) YIELD value
WHERE value.node IS NULL
WITH DISTINCT varDec, assStat, es.uniovi.reflection.codeanalysis.functions.getEnclosingCallable(varDec) as varMember
MATCH (varMember)<-[:DECLARES_METHOD| DECLARES_CONSTRUCTOR| TYPE_STATIC_INIT]-(enclClass)<-[:DECLARES_TYPE| HAS_NESTED_TYPE]-(cu:COMPILATION_UNIT)
RETURN '{"node":"ASSIGNMENT","line":"'+assStat.lineNumber+'","column":"'+assStat.column+'","rule":"CMU-OBJ54","location":"'+cu.fileName+'","message":"To make your code clearer, just delete the assignment in line '+assStat.lineNumber+' of the variable '+varDec.name+' declared in line '+varDec.lineNumber+', class '+enclClass.fullyQualifiedName+'."}'