This question already has answers here:
Close resource quietly using try-with-resources
(4 answers)
Closed 1 year ago.
I was reading about the try-with-resource in JDK7 and while I was thinking of upgrading my application to run with JDK7 I faced this problem..
When using a BufferedReader for example the write throws IOException and the close throws IOException.. in the catch block I am concerned in the IOException thrown by the write.. but I wouldn't care much about the one thrown by the close..
Same problem with database connections.. and any other resource..
As an example I've created an auto closeable resource:
public class AutoCloseableExample implements AutoCloseable {
public AutoCloseableExample() throws IOException{
throw new IOException();
}
#Override
public void close() throws IOException {
throw new IOException("An Exception During Close");
}
}
Now when using it:
public class AutoCloseTest {
public static void main(String[] args) throws Exception {
try (AutoCloseableExample example = new AutoCloseableExample()) {
System.out.println(example);
throw new IOException("An Exception During Read");
} catch (Exception x) {
System.out.println(x.getMessage());
}
}
}
how can I distinguish between such exceptions without having to create wrappers for classes such as BufferedReader?
Most of cases I put the resource close in a try/catch inside the finally block without caring much about handling it.
Lets consider the class:
public class Resource implements AutoCloseable {
public Resource() throws Exception {
throw new Exception("Exception from constructor");
}
public void doSomething() throws Exception {
throw new Exception("Exception from method");
}
#Override
public void close() throws Exception {
throw new Exception("Exception from closeable");
}
}
and the try-with-resource block:
try(Resource r = new Resource()) {
r.doSomething();
} catch (Exception ex) {
ex.printStackTrace();
}
1. All 3 throw statements enabled.
Message "Exception from constructor" will printed and the exception thrown by constructor will be suppressed, that means you can't catch it.
2. The throw in constructor is removed.
Now the stack trace will print "Exception from method" and "Suppressed: Exception from closeable" below. Here you also can't catch the suppressed exception thrown by close method, but you will be nofitied about the suppressed exception.
3. Throws from constructor and method are removed.
As you have probably already guessed the "Exception from closeable" will be printed.
Important tip: In all of above situations you are actually catching all exceptions, no matter where they were throwed. So if you use try-with-resource block you don't need to wrap the block with another try-catch, the extra block is simply useless.
Hope it helps :)
I would suggest using a flag as in the following example:
static String getData() throws IOException {
boolean isTryCompleted = false;
String theData = null;
try (MyResource br = new MyResource();) {
theData = br.getData();
isTryCompleted = true;
} catch(IOException e) {
if (!isTryCompleted )
throw e;
// else it's a close exception and it can be ignored
}
return theData;
}
source:Close resource quietly using try-with-resources
Related
This question already has answers here:
Try-with-resources: Must I throw or catch the close() method's exceptions? [duplicate]
(4 answers)
Closed last year.
In this code, when there is an exception in br.close(), will catch block catch it or will the process be terminated?
import java.io.BufferedReader;
import java.io.InputStreamReader;
public class TryWithBlock {
public static void main(String args[])
{
System.out.println("Enter a number");
try(BufferedReader br = new BufferedReader(new InputStreamReader(System.in)))
{
int i = Integer.parseInt(br.readLine());
System.out.println(i);
}
catch(Exception e)
{
System.out.println("A wild exception has been caught");
System.out.println(e);
}
}
}
From the docs:
The resource declared in the try-with-resources statement is a
BufferedReader. The declaration statement appears within parentheses
immediately after the try keyword. The class BufferedReader, in Java
SE 7 and later, implements the interface java.lang.AutoCloseable.
Because the BufferedReader instance is declared in a try-with-resource
statement, it will be closed regardless of whether the try statement
completes normally or abruptly (as a result of the method
BufferedReader.readLine throwing an IOException).
Basically, it's equivalent to:
BufferedReader br = new BufferedReader(new FileReader(System.in));
try {
int i = Integer.parseInt(br.readLine());
System.out.println(i);
} finally {
br.close();
}
Let's try it out (from here a working example):
//class implementing java.lang.AutoCloseable
public class ClassThatAutoCloses implements java.lang.AutoCloseable {
public ClassThatAutoCloses() {}
public void doSomething() {
System.out.println("Pippo!");
}
#Override
public void close() throws Exception {
throw new Exception("I wasn't supposed to fail");
}
}
//the main class
public class Playground {
/**
* #param args
*/
public static void main(String[] args) {
//this catches exceptions eventually thrown by the close
try {
try(var v = new ClassThatAutoCloses() ){
v.doSomething();
}
} catch (Exception e) {
//if something isn't catched by try()
//close failed will be printed
System.err.println("close failed");
e.printStackTrace();
}
}
}
//the output
Pippo!
close failed
java.lang.Exception: I wasn't supposed to fail
at ClassThatAutoCloses.close(ClassThatAutoCloses.java:26)
at Playground.main(Playground.java:24)
Exceptions thrown inside the try-with-resources statement are supressed. Exceptions thrown inside the try block are propagated. So in your case the catch block will catch the parsing exception (if any).
For more details you can refer at the docs.
i already know that the traditional Try block in java must have at least catch block or finally block (both or either), and i already know that checked exceptions must be handled or declared.
but i am wondering why it won't compile although i have used correct try block syntax
i have this piece of code here , in the main method i used Try with finally block but i am wondering why it won't compile
Here is my code:
import java.io.IOException;
import java.net.Socket;
public class ExHandling {
public void connect() throws IOException
{
Socket s = new Socket();
try
{
s.getInputStream();
}
catch(IOException e )
{
e.printStackTrace();
}
finally
{
s.close();
}
}
public static void main(String []args)
{
ExHandling ex = new ExHandling();
try
{
ex.connect();
}
finally
{
System.out.println("Finally");
}
}
}
Any Help Please
Remove the throws clause from your connect() method. It already catches the IOException. If you declare your method as throwing a checked exception it must be caught upon calling.
Update: since Socket#close() can itself throw an exception, you need to decide what do you want to do about it. Exception handling is hard because people tend to only think about the happiest path a program can take.
If you don't want to catch the exception explicitly in the main() method, you have only one choice: wrap the call to s.close() (and every other method that can throw a checked exception) into its' own try-catch block and remove the throws clause:
public void connect() {
Socket s = new Socket();
try {
s.getInputStream();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
s.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
But you should probably think—"what should I do when it fails?"—each time you're dealing with code that might throw.
Either catch the thrown IOException or throw it and let JVM handle the same.
IOException checked exception so you need catch or add an exception to a signature method. final just guarantee whatever happens final block will be executed.
Declare your main method to catch the IOException. If you do so, when the exception is thrown in your connect() method, it will be propagated to the main method and your finally block will be executed. If that is what you wanted.
finally itself cannot handle any exception. So when using try{} finally then the code inside try should either not be raising any exception or your method must be throwing the exception.
It won't compile because just as you indicated, the s.close() in your finally block can throw an IOException and you chose to handle that checked exception by specifying the "throws IOException" clause. Because of that choice, the calling method must handle that checked exception by either catching it or also specifying it will throw the exception.
It is unclear what results you desire other than it must compile, so here are three options:
1) Wrap s.close with it's own try/catch and remove the throws clause.
2) Move "ExHandling ex" definition inside the caller's try/catch.
3) Add a throws clause to the caller (and remove the try/finally if desired). RECOMMENDED
CAUTION: You really don't want to catch an exception and do "e.printStackTrace();". All this does is mask issues in your logic. You should only catch an exception if you plan to handle it in some manner; otherwise, you should allow the exception to propagate up the chain of callers. Thus, only use options 1 & 2 if you really wish to do something in all the catch clauses.
Option 1: Wrap s.close with it's own try/catch and remove the throws clause.
public void connect() {
Socket s = new Socket();
try {
s.getInputStream();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
s.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
ExHandling ex = new ExHandling();
try {
ex.connect();
}
finally {
System.out.println("Finally");
}
}
Option 2: Move "ExHandling ex" definition inside the caller's try/catch. In this case I would recommend using try with resources for the socket.
public void connect() throws IOException {
Socket s = new Socket();
s.getInputStream();
s.close();
}
public static void main(String[] args) {
try {
ExHandling ex = new ExHandling();
ex.connect();
} catch (IOException e) {
e.printStackTrace();
}
finally {
System.out.println("Finally");
}
}
Option 3: Add a throws clause to the caller (and remove the try/finally if desired). RECOMMENDED
public void connect() throws IOException {
Socket s = new Socket();
s.getInputStream();
s.close();
}
public static void main(String[] args) throws IOException {
ExHandling ex = new ExHandling();
ex.connect();
System.out.println("Finally");
}
Here's the code.
public class TestTest {
public static void main (String[] args) throws Exception {
try {
run();
} catch(Exception e) {
printSuppressedExceptions(e);
}
}
public static void printSuppressedExceptions(Throwable t) {
System.out.println(t);
System.out.println("suppressed exceptions: " + t.getSuppressed().length);
}
public static void run() throws Exception {
try(MyResource r = new MyResource("resource");) {
System.out.println("try");
System.getProperty("").length(); // throws illegalArgumentException
} catch(Exception e) {
printSuppressedExceptions(e);
throw e;
} finally {
new MyResource("finally").close();
}
}
}
class MyResource implements AutoCloseable {
private final String name;
public MyResource(String name) {
this.name = name;
}
#Override
public void close() throws Exception {
throw new Exception("exception" + " from " + this.name);
}
}
Since exception thrown from try block suppressed the exception from resource, I got "suppressed exceptions: 1" at first which was understandable. But when an exception was thrown from finally, it seemed like all suppressed exceptions disappeared because I got "java.lang.Exception: exception from finally" followed by "suppressed exceptions: 0" which I think it should be 1.
I browsed the Java tutorials and it definitely says
However, in this example, if the methods readLine and close both throw exceptions, then the method readFirstLineFromFileWithFinallyBlock throws the exception thrown from the finally block; the exception thrown from the try block is suppressed.
From The try-with-resources Statement
How could it happen?
Here is code that does what you would expect:
public class TestTest {
public static void main (String[] args) throws Exception {
try {
run();
} catch(Exception e) {
printSuppressedExceptions(e);
}
}
public static void printSuppressedExceptions(Throwable t) {
System.out.println(t);
System.out.println("suppressed exceptions (" + t.getSuppressed().length + "):");
for (Throwable suppressed : t.getSuppressed()) {
System.out.println(" - " + suppressed);
}
}
public static void run() throws Exception {
Exception exceptionFromCatch = null;
try(MyResource r = new MyResource("resource");) {
System.out.println("try");
System.getProperty("").length(); // throws illegalArgumentException
} catch(Exception e) {
exceptionFromCatch = e;
printSuppressedExceptions(e);
throw e;
} finally {
try {
new MyResource("finally").close();
} catch (Exception e) {
if (exceptionFromCatch!=null) {
e.addSuppressed(exceptionFromCatch);
}
throw e;
}
}
}
}
class MyResource implements AutoCloseable {
private final String name;
public MyResource(String name) {
this.name = name;
}
#Override
public void close() throws Exception {
throw new Exception("exception" + " from " + this.name);
}
}
So lets go trough the try-with-resource part of your code (as introduced in JDK 1.7.0) and see what happens (see What is the Java 7 try-with-resources bytecode equivalent using try-catch-finally? for more details):
the try-with-resource block MyResource r = new MyResource("resource") is executed
the try block is executed and throws an IllegalArgumentException
the try-with-resource block calls close() for all resources (in your example only one)
close() throws an exception, but since the exception from the try block has priority the exception from thrown by close() is suppressed and added via addSuppressed(..)
So that part works like you expected from reading the tutorial.
And now the try-catch-finally part of your code (as in JDK 1.6 and earlier):
the try block is executed and throws an IllegalArgumentException
(the catch block behaves the same way as if there was no catch block)
the finally block is executed and throws an Exception
the exception from the finally block has priority and the one from the try block is suppressed
But this time the word suppressed used in the java tutorial does not stand for "suppressed and added to the actually thrown exception" but "suppressed and lost to nirvana". So it still behaves as in JDK 1.6 and earlier and does not make use of the newly introduced addSuppressed(..) getSuppressed() functionality. That's the reason it doesn't behave like you expected.
I would argue the behaviour you expected wouldn't be logical either. I would like it to behave like this:
...
} finally {
try {
new MyResource("finally").close();
} catch (Exception e) {
if (exceptionFromCatch!=null) {
exceptionFromCatch.addSuppressed(e);
} else {
throw e;
}
}
}
...
That would always give priority to the exception from the try block (as implemented with the new try-with-resource feature) and add the exception from the catch block as suppressed to the list. But that would break compatibility with JDK 1.6, so I guess that's the reason why it doesn't behave like that.
A coworker just unsettled me concerning finally blocks. He claimed that if multiple resources are closed inside a finally block, I do not have to worry about exception handling.
So if I close my resources like this
try {
// do stuff
} catch(Exception e) {
// handle stuff
} finally {
resource1.close();
resource2.close();
}
and an exception occurs at resource1.close(), will the close() method of resource2 get called?
A simple check would confirm:
class MyResource implements AutoCloseable {
private final String name;
MyResource(String name) { this.name = name; }
#Override public void close() throws IOException {
System.out.println("Closing " + name);
throw new IOException();
}
}
public static void main(String[] args) throws IOException {
MyResource a = new MyResource("a");
MyResource b = new MyResource("b");
try {
} finally {
a.close();
b.close();
}
}
This would print "Closing a" and then print a stack trace; "Closing b" would not be printed. In contrast:
try (MyResource a = new MyResource("a");
MyResource b = new MyResource("b")) {
}
would print both.
That depends. If the only exception throwing things (explicitly or potentially) you have inside your try-catch block are close operations, you wouldn't need exception handling. However, most of the times, the close operations are themselves declared as throwing exceptions, thus, you'd need to put them inside a try-catch block anyway.
import java.io.*;
class West1 extends Exception {
private String msg;
public West1() {
}
public West1(String msg) {
super(msg);
this.msg=msg;
}
public West1(Throwable cause) {
super(cause);
}
public West1(String msg,Throwable cause) {
super(msg,cause);
this.msg=msg;
}
public String toString() {
return msg;
}
public String getMessage() {
return msg;
}
}
public class West {
public static void main(String[] args) {
try {
throw new West1("Custom Exception.....");
}catch(West1 ce) {
System.out.println(ce.getMessage());
//throw new NumberFormatException();
throw new FileNotFoundException();
}catch(FileNotFoundException fne) {
fne.printStackTrace();
}/*catch(NumberFormatException nfe) {
nfe.printStackTrace();
}*/
}
}
In the above code, NumberFormatException is thrown from catch block it compile and run successfully but when FileNotFoundException is thrown from catch block it will not compile. Following Errors are thrown:
West.java:40: error: exception FileNotFoundException is never thrown in body of
corresponding try statement
}catch(FileNotFoundException fne){
West.java:39: error: unreported exception FileNotFoundException; must be caught
or declared to be thrown
throw new FileNotFoundException();
So my question is what is reason behind this behaviour?
NumberFormatException is a RuntimeException, meaning that it's not required to be declared in all methods it may be thrown. This means that, unlike FileNotFoundException, the compiler can not know if it can get thrown in a block or not.
The title implies you are trying to catch multiple exceptions at once, and your code implies you understand that you can have multiple catch blocks for a single try block to catch different types of exceptions. Everything is good so far, but you seem to misunderstand exactly where the try-catch catches errors.
Here is you code. I removed the comments to make it more concise.
public class West {
public static void main(String[] args) {
try {
throw new West1("Custom Exception.....");
} catch(West1 ce) {
System.out.println(ce.getMessage());
throw new FileNotFoundException(); // <-- Must be caught
} catch(FileNotFoundException fne) { // <-- Never thrown
fne.printStackTrace();
}
}
}
The first compiler error is because the catch block that is for catching FileNotFoundException's try block never throws FileNotFoundException. The second compiler error is because FileNotFoundException is a checked exception. Because it is a checked exception your code must either
handle it (by catching it with try-catch)
let everyone know it could throw it (public static void main(String[] args) throws FileNotFoundException { ...).
From the context of your code, you seem to be going with the first option, handling it with try-catch, but you have the catch in the wrong place.
catch blocks don't catch exceptions, try blocks do. catch blocks specify what to do with the actual caught exception.
public class West {
public static void main(String[] args) {
try {
throw new West1("Custom Exception.....");
} catch(West1 ce) {
System.out.println(ce.getMessage());
try {
throw new FileNotFoundException();
} catch(FileNotFoundException fne) {
fne.printStackTrace();
}
}
}
}
Try that instead. Notice how you can have a try instead of a catch. This wouldn't matter if FileNotFoundException wasn't a checked exception.