Considering this code, can I be absolutely sure that the finally
block always executes, no matter what something()
is?
try {
something();
return success;
}
catch (Exception e) {
return failure;
}
finally {
System.out.println("I don't know if this will get printed out");
}
finally
; finalizer == the finalize()
method. - anyone Yes, finally
will be called after the execution of the try
or catch
code blocks.
The only times finally
won't be called are:
System.exit()
Runtime.getRuntime().halt(exitStatus)
try
or catch
blockkill -9 <pid>
on UNIXfinally
block is going to be executed by a daemon thread and all other non-daemon threads exit before finally
is calledAnswered 2023-09-20 20:26:30
thread.stop()
does not necessarily prevent finally
block from being executed. - anyone finally
block will be called after the try
block, and before control passes to the following statements. That's consistent with the try block involving an infinite loop and hence the finally block never actually being invoked. - anyone Example code:
public static void main(String[] args) {
System.out.println(Test.test());
}
public static int test() {
try {
return 0;
}
finally {
System.out.println("something is printed");
}
}
Output:
something is printed.
0
Answered 2023-09-20 20:26:30
finally
-clause with return 2;
is not allowed (Compiler-Error). - anyone Also, although it's bad practice, if there is a return statement within the finally block, it will trump any other return from the regular block. That is, the following block would return false:
try { return true; } finally { return false; }
Same thing with throwing exceptions from the finally block.
Answered 2023-09-20 20:26:30
Here's the official words from the Java Language Specification.
14.20.2. Execution of try-finally and try-catch-finally
A
try
statement with afinally
block is executed by first executing thetry
block. Then there is a choice:
- If execution of the
try
block completes normally, [...]- If execution of the
try
block completes abruptly because of athrow
of a value V, [...]- If execution of the
try
block completes abruptly for any other reason R, then thefinally
block is executed. Then there is a choice:
- If the finally block completes normally, then the
try
statement completes abruptly for reason R.- If the
finally
block completes abruptly for reason S, then thetry
statement completes abruptly for reason S (and reason R is discarded).
The specification for return
actually makes this explicit:
JLS 14.17 The return Statement
ReturnStatement: return Expression(opt) ;
A
return
statement with noExpression
attempts to transfer control to the invoker of the method or constructor that contains it.A
return
statement with anExpression
attempts to transfer control to the invoker of the method that contains it; the value of theExpression
becomes the value of the method invocation.The preceding descriptions say "attempts to transfer control" rather than just "transfers control" because if there are any
try
statements within the method or constructor whosetry
blocks contain thereturn
statement, then anyfinally
clauses of thosetry
statements will be executed, in order, innermost to outermost, before control is transferred to the invoker of the method or constructor. Abrupt completion of afinally
clause can disrupt the transfer of control initiated by areturn
statement.
Answered 2023-09-20 20:26:30
In addition to the other responses, it is important to point out that 'finally' has the right to override any exception/returned value by the try..catch block. For example, the following code returns 12:
public static int getMonthsInYear() {
try {
return 10;
}
finally {
return 12;
}
}
Similarly, the following method does not throw an exception:
public static int getMonthsInYear() {
try {
throw new RuntimeException();
}
finally {
return 12;
}
}
While the following method does throw it:
public static int getMonthsInYear() {
try {
return 12;
}
finally {
throw new RuntimeException();
}
}
Answered 2023-09-20 20:26:30
OutOfMemoryError
? ;) - anyone return retVal
after the finally
block, although that of course assumes that you suppressed some other exceptions because the code would not make sense otherwise. - anyone Here's an elaboration of Kevin's answer. It's important to know that the expression to be returned is evaluated before finally
, even if it is returned after.
public static void main(String[] args) {
System.out.println(Test.test());
}
public static int printX() {
System.out.println("X");
return 0;
}
public static int test() {
try {
return printX();
}
finally {
System.out.println("finally trumps return... sort of");
return 42;
}
}
Output:
X
finally trumps return... sort of
42
Answered 2023-09-20 20:26:30
finally
. Calculating the return value (printX()
here) still comes before it. - anyone System.out.println("finally trumps return... sort of");
with System.out.print("finally trumps return in try"); return 42;
- anyone return
doesn't return some magic continuation that only gets evaluated if the caller prints it or whatever. printX()
gets called before the return
happens, regardless of any try
/catch
, or anything else. - anyone I tried the above example with slight modification-
public static void main(final String[] args) {
System.out.println(test());
}
public static int test() {
int i = 0;
try {
i = 2;
return i;
} finally {
i = 12;
System.out.println("finally trumps return.");
}
}
The above code outputs:
finally trumps return.
2
This is because when return i;
is executed i
has a value 2. After this the finally
block is executed where 12 is assigned to i
and then System.out
out is executed.
After executing the finally
block the try
block returns 2, rather than returning 12, because this return statement is not executed again.
If you will debug this code in Eclipse then you'll get a feeling that after executing System.out
of finally
block the return
statement of try
block is executed again. But this is not the case. It simply returns the value 2.
Answered 2023-09-20 20:26:30
i
was not a primitive, but an Integer object. - anyone That is the whole idea of a finally block. It lets you make sure you do cleanups that might otherwise be skipped because you return, among other things, of course.
Finally gets called regardless of what happens in the try block (unless you call System.exit(int)
or the Java Virtual Machine kicks out for some other reason).
Answered 2023-09-20 20:26:30
A logical way to think about this is:
Answered 2023-09-20 20:26:30
finally
is always executed unless there is abnormal program termination (like calling System. exit(0)
). so, your System.out
will get printed.
Answered 2023-09-20 20:26:30
No, not always, one exception case is
System.exit(0);
before the finally
block prevents finally
to be executed.
class A {
public static void main(String args[]) {
DataInputStream cin = new DataInputStream(System.in);
try {
int i = Integer.parseInt(cin.readLine());
} catch (ArithmeticException e) {
} catch (Exception e) {
System.exit(0); // Program terminates before executing the finally block
} finally {
System.out.println("Won't be executed");
System.out.println("No error");
}
}
}
Answered 2023-09-20 20:26:30
Also a return in finally will throw away any exception. http://jamesjava.blogspot.com/2006/03/dont-return-in-finally-clause.html
Answered 2023-09-20 20:26:30
The finally block is always executed unless there is abnormal program termination, either resulting from a JVM crash or from a call to System.exit(0)
.
On top of that, any value returned from within the finally block will override the value returned prior to execution of the finally block, so be careful of checking all exit points when using try finally.
Answered 2023-09-20 20:26:30
Finally is always run that's the whole point, just because it appears in the code after the return doesn't mean that that's how it's implemented. The Java runtime has the responsibility to run this code when exiting the try
block.
For example if you have the following:
int foo() {
try {
return 42;
}
finally {
System.out.println("done");
}
}
The runtime will generate something like this:
int foo() {
int ret = 42;
System.out.println("done");
return 42;
}
If an uncaught exception is thrown the finally
block will run and the exception will continue propagating.
Answered 2023-09-20 20:26:30
The Java Language specification describes how try
-catch
-finally
and try
-catch
blocks work at 14.20.2
In no place it specifies that the finally
block is always executed.
But for all cases in which the try
-catch
-finally
and try
-finally
blocks complete it does specify that before completion finally
must be executed.
try {
CODE inside the try block
}
finally {
FIN code inside finally block
}
NEXT code executed after the try-finally block (may be in a different method).
The JLS does not guarantee that FIN is executed after CODE. The JLS guarantees that if CODE and NEXT are executed then FIN will always be executed after CODE and before NEXT.
Why doesn't the JLS guarantee that the finally
block is always executed after the try
block? Because it is impossible. It is unlikely but possible that the JVM will be aborted (kill, crash, power off) just after completing the try
block but before execution of the finally
block. There is nothing the JLS can do to avoid this.
Thus, any software which for their proper behaviour depends on finally
blocks always being executed after their try
blocks complete are bugged.
return
instructions in the try
block are irrelevant to this issue. If execution reaches code after the try
-catch
-finally
it is guaranteed that the finally
block will have been executed before, with or without return
instructions inside the try
block.
Answered 2023-09-20 20:26:30
any software which for their proper behaviour depends on finally blocks always being executed after their try blocks complete are bugged.
is quite right. finally
blocks are usually used for resource clean-up to prevent leaks. How else would you do it? In the same vein, you cannot (or should not...) catch Error
s either, as there is (usually) no reasonable way for a normal application to handle them. - anyone finally
may (given enough time, likely will) cause bugs, but is the additional development (and testing) time worth the potentially unnoticeable improvement in stability? More generally: does the amount of care that should go into a particular design depend on the subsystem in question? Time is a valuable resource, after all. :) - anyone Yes it will get called. That's the whole point of having a finally keyword. If jumping out of the try/catch block could just skip the finally block it was the same as putting the System.out.println outside the try/catch.
Answered 2023-09-20 20:26:30
Because a finally block will always be called unless you call System.exit()
(or the thread crashes).
Answered 2023-09-20 20:26:30
Concisely, in the official Java Documentation (Click here), it is written that -
If the JVM exits while the try or catch code is being executed, then the finally block may not execute. Likewise, if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application as a whole continues.
Answered 2023-09-20 20:26:30
This is because you assigned the value of i as 12, but did not return the value of i to the function. The correct code is as follows:
public static int test() {
int i = 0;
try {
return i;
} finally {
i = 12;
System.out.println("finally trumps return.");
return i;
}
}
Answered 2023-09-20 20:26:30
finally
block is always executed and before returning x
's (calculated) value.
System.out.println("x value from foo() = " + foo());
...
int foo() {
int x = 2;
try {
return x++;
} finally {
System.out.println("x value in finally = " + x);
}
}
Output:
x value in finally = 3
x value from foo() = 2
Answered 2023-09-20 20:26:30
Answer is simple YES.
INPUT:
try{
int divideByZeroException = 5 / 0;
} catch (Exception e) {
System.out.println("catch");
return; // also tried with break; in switch-case, got same output
} finally {
System.out.println("finally");
}
OUTPUT:
catch
finally
Answered 2023-09-20 20:26:30
Yes, it will. No matter what happens in your try or catch block unless otherwise System.exit() called or JVM crashed. if there is any return statement in the block(s),finally will be executed prior to that return statement.
Answered 2023-09-20 20:26:30
Adding to @vibhash's answer as no other answer explains what happens in the case of a mutable object like the one below.
public static void main(String[] args) {
System.out.println(test().toString());
}
public static StringBuffer test() {
StringBuffer s = new StringBuffer();
try {
s.append("sb");
return s;
} finally {
s.append("updated ");
}
}
Will output
sbupdated
Answered 2023-09-20 20:26:30
Yes, finally block is always execute. Most of developer use this block the closing the database connection, resultset object, statement object and also uses into the java hibernate to rollback the transaction.
Answered 2023-09-20 20:26:30
finally
will execute and that is for sure.
finally
will not execute in below cases:
case 1 :
When you are executing System.exit()
.
case 2 :
When your JVM / Thread crashes.
case 3 :
When your execution is stopped in between manually.
Answered 2023-09-20 20:26:30
I tried this, It is single threaded.
public static void main(String args[]) throws Exception {
Object obj = new Object();
try {
synchronized (obj) {
obj.wait();
System.out.println("after wait()");
}
} catch (Exception ignored) {
} finally {
System.out.println("finally");
}
}
The main
Thread
will be on wait
state forever, hence finally
will never be called,
so console output will not print
String
: after wait()
or finally
Agreed with @Stephen C, the above example is one of the 3rd case mention here:
Adding some more such infinite loop possibilities in following code:
// import java.util.concurrent.Semaphore;
public static void main(String[] args) {
try {
// Thread.sleep(Long.MAX_VALUE);
// Thread.currentThread().join();
// new Semaphore(0).acquire();
// while (true){}
System.out.println("after sleep join semaphore exit infinite while loop");
} catch (Exception ignored) {
} finally {
System.out.println("finally");
}
}
Case 2: If the JVM crashes first
import sun.misc.Unsafe;
import java.lang.reflect.Field;
public static void main(String args[]) {
try {
unsafeMethod();
//Runtime.getRuntime().halt(123);
System.out.println("After Jvm Crash!");
} catch (Exception e) {
} finally {
System.out.println("finally");
}
}
private static void unsafeMethod() throws NoSuchFieldException, IllegalAccessException {
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);
unsafe.putAddress(0, 0);
}
Case 6: If finally
block is going to be executed by daemon Thread
and all other non-daemon Threads
exit before finally
is called.
public static void main(String args[]) {
Runnable runnable = new Runnable() {
@Override
public void run() {
try {
printThreads("Daemon Thread printing");
// just to ensure this thread will live longer than main thread
Thread.sleep(10000);
} catch (Exception e) {
} finally {
System.out.println("finally");
}
}
};
Thread daemonThread = new Thread(runnable);
daemonThread.setDaemon(Boolean.TRUE);
daemonThread.setName("My Daemon Thread");
daemonThread.start();
printThreads("main Thread Printing");
}
private static synchronized void printThreads(String str) {
System.out.println(str);
int threadCount = 0;
Set<Thread> threadSet = Thread.getAllStackTraces().keySet();
for (Thread t : threadSet) {
if (t.getThreadGroup() == Thread.currentThread().getThreadGroup()) {
System.out.println("Thread :" + t + ":" + "state:" + t.getState());
++threadCount;
}
}
System.out.println("Thread count started by Main thread:" + threadCount);
System.out.println("-------------------------------------------------");
}
output: This does not print "finally" which implies "Finally block" in "daemon thread" did not execute
main Thread Printing Thread :Thread[My Daemon Thread,5,main]:state:BLOCKED Thread :Thread[main,5,main]:state:RUNNABLE Thread :Thread[Monitor Ctrl-Break,5,main]:state:RUNNABLE Thread count started by Main thread:3 ------------------------------------------------- Daemon Thread printing Thread :Thread[My Daemon Thread,5,main]:state:RUNNABLE Thread :Thread[Monitor Ctrl-Break,5,main]:state:RUNNABLE Thread count started by Main thread:2 ------------------------------------------------- Process finished with exit code 0
Answered 2023-09-20 20:26:30
Consider the following program:
public class SomeTest {
private static StringBuilder sb = new StringBuilder();
public static void main(String args[]) {
System.out.println(someString());
System.out.println("---AGAIN---");
System.out.println(someString());
System.out.println("---PRINT THE RESULT---");
System.out.println(sb.toString());
}
private static String someString() {
try {
sb.append("-abc-");
return sb.toString();
} finally {
sb.append("xyz");
}
}
}
As of Java 1.8.162, the above code block gives the following output:
-abc-
---AGAIN---
-abc-xyz-abc-
---PRINT THE RESULT---
-abc-xyz-abc-xyz
this means that using finally
to free up objects is a good practice like the following code:
private static String someString() {
StringBuilder sb = new StringBuilder();
try {
sb.append("abc");
return sb.toString();
} finally {
sb = null; // Just an example, but you can close streams or DB connections this way.
}
}
Answered 2023-09-20 20:26:30
sb.setLength(0)
in finally? - anyone sb = null;
just adds unneeded code. I understand that you mean that a finally
block is a good place to free resources like a database connection or something like that, but have in mind that your example could confuse newcomers. - anyone System.out.println("---AGAIN2---");
System.out.println(sb);
and it's more clear now. As it was, the output was against your thesis :p I also added to your answer, but the edit must be accepted by a moderator or somebody like that. Else you can add them - anyone That's actually true in any language...finally will always execute before a return statement, no matter where that return is in the method body. If that wasn't the case, the finally block wouldn't have much meaning.
Answered 2023-09-20 20:26:30
In addition to the point about return in finally replacing a return in the try block, the same is true of an exception. A finally block that throws an exception will replace a return or exception thrown from within the try block.
Answered 2023-09-20 20:26:30