RLK (Resource Leak) issues are reported when resources are allocated but not properly disposed after use. Failing to properly dispose a resource can lead to such problems as:

too many files being open
an application not being able to access a temporary file when it is needed

RLK.OUT indicates that an output stream that was opened is not explicitly closed.

Vulnerability and risk

Resources such as streams, connections and graphic objects must be explicitly closed. The close operation can unblock transactions or flush file changes in the file system. While a resource will eventually be closed by the garbage collector, resource exhaustion can occur before garbage collection starts. Depending on the nature of the resource, various exceptions will be thrown on a failed attempt to allocate another resource, for example: java.io.FileNotFoundException: Too many open files or too many database connections.

Mitigation and prevention

Explicitly close all resources that have the close method, even those that you think are not doing anything significant. Future code changes will then be safe from such errors.

Example 1

11     public void printLines(String fName, Collection<String> lines) {
12         try {
13             File file = new File(fName);
14             PrintWriter pstr = new PrintWriter(
15                     new BufferedOutputStream(
16                             new FileOutputStream(file)));
17             for (final String line : lines) {
18                 pstr.println(line);
19             }
20         } catch (IOException e) {
21             e.printStackTrace();
22         }
23     }

RLK.OUT is reported for the snippet on line 16: Output stream 'pstr' is not closed on exit.

Example 2

11    public void printLines(String fName, Collection<String> lines) {
12         try {
13             File file = new File(fName);
14             PrintWriter pstr = new PrintWriter(
15                     new BufferedOutputStream(
16                             new FileOutputStream(file)));
17             try {
18                 for (final String line : lines) {
19                     pstr.println(line);
20                 }
21             } finally {
22                 pstr.close();
23             }
24         } catch (IOException e) {
25             e.printStackTrace();
26         }
27     }

The snippet from the previous section is fixed; RLK.OUT is not reported here.

Security guidelines

CWE-404: Improper Resource Shutdown or Release

Extension

This checker can be extended through the Klocwork knowledge base. See Tuning Java analysis for more information.