The following code uses the try
-with-resources construction introduced in Java 8. The occasionallyThrow() method is declared to throw an OccasionalException, the Resource's close() method to throw a CloseException. Eclipse (Version: Neon Release (4.6.0), Build id: 20160613-1800) adds a warning on the line marked with // dead code that the branch is dead code. Implicitly, Eclipse affirms that the line marked with // alive code is not dead code.
Object tryWithResources() throws OccasionalException {
Object value = null;
try (Resource resource = new Resource()) {
occasionallyThrow();
value = new Object();
}
catch (CloseException e) {
if (value == null) {
// alive code
}
else {
// dead code
}
}
return value;
}
I'm confused by this. If occasionallyThrow() throws its OccasionalException, then the try
-with-resources should catch that as the primary exception and then attempt to close the resource. If closing the resource throws a CloseException, then it will be suppressed under the OccasionalException, so there won't be the CloseException to catch. So, the only time there should be CloseException to catch is when the block within the try completed successfully which means that value is non-null. So it seems like the "dead code" is actually alive, and the "alive code" is actually dead. I'm not sure what a compiler is actually expected to recognize here, but at the very least, it seems like the "dead code" here should not be called dead.
What makes this more complicated is that the translated form without using the try-with-resources form doesn't get marked with any dead code warnings at all. (I'm fairly confident that I got this translation right, based on 14.20.3.2. Extended try-with-resources, but I wouldn't be completely surprised if there's bug here…)
Object expandedTry() throws OccasionalException {
Object value = null;
try {
Resource resource = new Resource();
Throwable $primary = null;
try {
occasionallyThrow();
value = new Object();
}
catch (Throwable t) {
$primary = t;
throw t;
}
finally {
if (resource != null) {
if ($primary != null) {
try {
resource.close();
}
catch (Throwable $suppressed) {
$primary.addSuppressed($suppressed);
}
}
else {
resource.close();
}
}
}
}
catch (CloseException e) {
if (value == null) {
// alive (not dead!)
}
else {
// alive
}
}
return value;
}
Am I missing something that would make either branch in the if-else dead in one of these, but not in the other?
Full Code
Here's the full code with definitions of the auxiliary exception types, the Resource class, and the top-level class.
public class TestTryWithResources {
/** Exception thrown by Resource's close() method */
@SuppressWarnings("serial")
static class CloseException extends Exception {}
/** AutoCloseable declared to throw a CloseException */
static class Resource implements AutoCloseable {
@Override
public void close() throws CloseException {}
}
/** An occasionally thrown exception */
@SuppressWarnings("serial")
static class OccasionalException extends Exception {}
/** Method declared to throw an occasional exception */
void occasionallyThrow() throws OccasionalException {}
/*
* Method using try-with-resources. Eclipse warns that the
* portion marked with "// dead code" is Dead code.
*/
Object tryWithResources() throws OccasionalException {
Object value = null;
try (Resource resource = new Resource()) {
occasionallyThrow();
value = new Object();
}
catch (CloseException e) {
if (value == null) {
// alive code
}
else {
// dead code
}
}
return value;
}
/*
* Method not using try-with-resources. This is the translation
* of the try-with-resources in tryWithResources, according to
* [14.20.3 try-with-resources][1]. Eclipse does not warn about
* any of the code being Dead code.
*
* [1]: https://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.20.3
*/
Object expandedTry() throws OccasionalException {
Object value = null;
try {
Resource resource = new Resource();
Throwable $primary = null;
try {
occasionallyThrow();
value = new Object();
}
catch (Throwable t) {
$primary = t;
throw t;
}
finally {
if (resource != null) {
if ($primary != null) {
try {
resource.close();
}
catch (Throwable $suppressed) {
$primary.addSuppressed($suppressed);
}
}
else {
resource.close();
}
}
}
}
catch (CloseException e) {
if (value == null) {
// alive
}
else {
// alive
}
}
return value;
}
}
Responses to Comments
Amin J's answer suggested a workaround of using the resource after setting value to change Eclipse's code analysis. This doesn't work though. After using the resource, e.g., by printing it, the dead code warning is still present in both Luna and Neon: