My instance revealed just one trigger. Exceptions thrown from non-trivial real-world purposes might include intensive chains of many causes. You may entry these causes by using a loop equivalent to the next:
catch (Exception exc)
{
Throwable t = exc.getCause();
whereas (t != null)
{
System.out.println(t);
t = t.getCause();
}
}Attempt-with-resources
Java purposes typically entry information, database connections, sockets, and different sources that rely upon associated system sources (e.g., file handles). The shortage of system sources implies that they have to finally be launched, even when an exception happens. When system sources aren’t launched, the applying finally fails when making an attempt to amass different sources, as a result of no extra associated system sources can be found.
In my introduction to exception dealing with fundamentals, I discussed that sources (really, the system sources on which they rely) are launched in a lastly block. This will result in tedious boilerplate code, such because the file-closing code that seems beneath:
lastly
{
if (fis != null)
attempt
{
fis.shut();
}
catch (IOException ioe)
{
// ignore exception
}
if (fos != null)
attempt
{
fos.shut();
}
catch (IOException ioe)
{
// ignore exception
}
}Not solely does this boilerplate code add bulk to a classfile, the tedium of writing it’d result in a bug, maybe even failing to shut a file. JDK 7 launched try-with-resources to beat this drawback.
The fundamentals of try-with-resources
The try-with-resources assemble mechanically closes open sources when execution leaves the scope through which they have been opened and used, whether or not or not an exception is thrown from that scope. This assemble has the next syntax:
attempt (useful resource acquisitions)
{
// useful resource utilization
}The attempt key phrase is parameterized by a semicolon-separated record of resource-acquisition statements, the place every assertion acquires a useful resource. Every acquired useful resource is obtainable to the physique of the attempt block, and is mechanically closed when execution leaves this physique. In contrast to a daily attempt assertion, try-with-resources doesn’t require catch blocks and/or a lastly block to observe attempt(), though they are often specified.
Think about the next file-oriented instance:
attempt (FileInputStream fis = new FileInputStream("abc.txt"))
{
// Do one thing with fis and the underlying file useful resource.
}On this instance, an enter stream to an underlying file useful resource (abc.txt) is acquired. The attempt block does one thing with this useful resource, and the stream (and file) is closed upon exit from the attempt block.
Utilizing ‘var’ with ‘try-with-resources’
JDK 10 launched help for var, an identifier with particular which means (i.e., not a key phrase). You should utilize var with try-with-resources to scale back boilerplate. For instance, you could possibly simplify the earlier instance to the next:
attempt (var fis = new FileInputStream("abc.txt"))
{
// Do one thing with fis and the underlying file useful resource.
}Copying a file in a try-with-resources context
Within the earlier article, I excerpted the copy() technique from a file-copy software. This technique’s lastly block accommodates the file-closing boilerplate introduced earlier. Itemizing 8 improves this technique by utilizing try-with-resources to deal with the cleanup.
Itemizing 8. Copy.java
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
public class Copy
{
public static void principal(String[] args)
{
if (args.size != 2)
{
System.err.println("utilization: java Copy srcfile dstfile");
return;
}
attempt
{
copy(args[0], args[1]);
}
catch (IOException ioe)
{
System.err.println("I/O error: " + ioe.getMessage());
}
}
static void copy(String srcFile, String dstFile) throws IOException
{
attempt (FileInputStream fis = new FileInputStream(srcFile);
FileOutputStream fos = new FileOutputStream(dstFile))
{
int c;
whereas ((c = fis.learn()) != -1)
fos.write(c);
}
}
}copy() makes use of try-with-resources to handle supply and vacation spot file sources. The spherical bracketed-code following attempt makes an attempt to create file enter and output streams to those information. Assuming success, its physique executes, copying the supply file to the vacation spot file.
Whether or not an exception is thrown or not, try-with-resources ensures that each information are closed when execution leaves the attempt block. As a result of the boilerplate file-closing code that was proven earlier isn’t wanted, Itemizing 8’s copy() technique is way less complicated and simpler to learn.
Designing useful resource lessons to help try-with-resources
The try-with-resources assemble requires {that a} useful resource class implement the java.lang.Closeable interface or the JDK 7-introduced java.lang.AutoCloseable superinterface. Pre-Java 7 lessons like java.io.FileInputStream applied Closeable, which affords a void shut() technique that throws IOException or a subclass.
Beginning with Java 7, lessons can implement AutoCloseable, whose single void shut() technique can throw java.lang.Exception or a subclass. The throws clause has been expanded to accommodate conditions the place you would possibly want so as to add shut() strategies that may throw exceptions outdoors of the IOException hierarchy; for instance, java.sql.SQLException.
Itemizing 9 presents a CustomARM software that reveals you how one can configure a customized useful resource class so that you could use it in a try-with-resources context.
Itemizing 9. CustomARM.java
public class CustomARM
{
public static void principal(String[] args)
{
attempt (USBPort usbp = new USBPort())
{
System.out.println(usbp.getID());
}
catch (USBException usbe)
{
System.err.println(usbe.getMessage());
}
}
}
class USBPort implements AutoCloseable
{
USBPort() throws USBException
{
if (Math.random() < 0.5)
throw new USBException("unable to open port");
System.out.println("port open");
}
@Override
public void shut()
{
System.out.println("port shut");
}
String getID()
{
return "some ID";
}
}
class USBException extends Exception
{
USBException(String msg)
{
tremendous(msg);
}
}Itemizing 9 simulates a USB port in which you’ll be able to open and shut the port and return the port’s ID. I’ve employed Math.random() within the constructor so that you could observe try-with-resources when an exception is thrown or not thrown.
Compile this itemizing and run the applying. If the port is open, you’ll see the next output:
port open
some ID
port shutIf the port is closed, you would possibly see this:
unable to open portSuppressing exceptions in try-with-resources
In case you’ve had some programming expertise, you may need seen a possible drawback with try-with-resources: Suppose the attempt block throws an exception. This assemble responds by invoking a useful resource object’s shut() technique to shut the useful resource. Nevertheless, the shut() technique may additionally throw an exception.
When shut() throws an exception (e.g., FileInputStream‘s void shut() technique can throw IOException), this exception masks or hides the unique exception. Plainly the unique exception is misplaced.
In truth, this isn’t the case: try-with-resources suppresses shut()‘s exception. It additionally provides the exception to the unique exception’s array of suppressed exceptions by invoking java.lang.Throwable‘s void addSuppressed(Throwable exception) technique.
Itemizing 10 presents a SupExDemo software that demonstrates how one can repress an exception in a try-with-resources context.
Itemizing 10. SupExDemo.java
import java.io.Closeable;
import java.io.IOException;
public class SupExDemo implements Closeable
{
@Override
public void shut() throws IOException
{
System.out.println("shut() invoked");
throw new IOException("I/O error in shut()");
}
public void doWork() throws IOException
{
System.out.println("doWork() invoked");
throw new IOException("I/O error in work()");
}
public static void principal(String[] args) throws IOException
{
attempt (SupExDemo supexDemo = new SupExDemo())
{
supexDemo.doWork();
}
catch (IOException ioe)
{
ioe.printStackTrace();
System.out.println();
System.out.println(ioe.getSuppressed()[0]);
}
}
}Itemizing 10’s doWork() technique throws an IOException to simulate some type of I/O error. The shut() technique additionally throws the IOException, which is suppressed in order that it doesn’t masks doWork()‘s exception.
The catch block accesses the suppressed exception (thrown from shut()) by invoking Throwable‘s Throwable[] getSuppressed() technique, which returns an array of suppressed exceptions. Solely the primary component is accessed as a result of just one exception is suppressed.
Compile Itemizing 10 and run the applying. You need to observe the next output:
doWork() invoked
shut() invoked
java.io.IOException: I/O error in work()
at SupExDemo.doWork(SupExDemo.java:16)
at SupExDemo.principal(SupExDemo.java:23)
Suppressed: java.io.IOException: I/O error in shut()
at SupExDemo.shut(SupExDemo.java:10)
at SupExDemo.principal(SupExDemo.java:24)
java.io.IOException: I/O error in shut()A number of catch blocks (multi-catch)
Beginning in JDK 7, it’s doable to codify a single catch block that catches multiple kind of exception. The aim of this multi-catch function is to scale back code duplication and cut back the temptation to catch overly broad exceptions (as an illustration, catch (Exception e)).
Suppose you’ve developed an software that offers you the pliability to repeat knowledge to a database or file. Itemizing 11 presents a CopyToDatabaseOrFile class that simulates this case, and demonstrates the issue with catch block code duplication.
Itemizing 11. CopyToDatabaseOrFile.java
import java.io.IOException;
import java.sql.SQLException;
public class CopyToDatabaseOrFile
{
public static void principal(String[] args)
{
attempt
{
copy();
}
catch (IOException ioe)
{
System.out.println(ioe.getMessage());
// extra handler code
}
catch (SQLException sqle)
{
System.out.println(sqle.getMessage());
// extra handler code that is equivalent to the earlier handler's
// code
}
}
static void copy() throws IOException, SQLException
{
if (Math.random() < 0.5)
throw new IOException("can't copy to file");
else
throw new SQLException("can't copy to database");
}
}JDK 7 overcomes this code duplication drawback by letting you specify a number of exception varieties in a catch block the place every successive kind is separated from its predecessor by putting a vertical bar (|) between these varieties:
attempt
{
copy();
}
catch (IOException | SQLException iosqle)
{
System.out.println(iosqle.getMessage());
}Now, when copy() throws both exception, the exception can be caught and dealt with by the catch block.
When a number of exception varieties are listed in a catch block’s header, the parameter is implicitly thought to be ultimate. In consequence, you can’t change the parameter’s worth. For instance, you can’t change the reference saved within the earlier code fragment’s iosqle parameter.
Remaining re-throw
Beginning in JDK 7, the Java compiler is ready to analyze re-thrown exceptions extra exactly than in earlier Java variations. This function solely works when no assignments are made to a re-thrown exception’s catch block parameter, which is taken into account to be successfully ultimate. When a previous attempt block throws an exception that’s a supertype/subtype of the parameter’s kind, the compiler throws the caught exception’s precise kind as an alternative of throwing the parameter’s kind (as was achieved in earlier Java variations).
The aim of this ultimate re-throw function is to facilitate including a attempt–catch assertion round a block of code to intercept, course of, and re-throw an exception with out affecting the statically decided set of exceptions thrown from the code. Additionally, this function permits you to present a standard exception handler to partially deal with the exception near the place it’s thrown, and supply extra exact handlers elsewhere that deal with the re-thrown exception. Think about Itemizing 12.
Itemizing 12. MonitorEngine.java
class PressureException extends Exception
{
PressureException(String msg)
{
tremendous(msg);
}
}
class TemperatureException extends Exception
{
TemperatureException(String msg)
{
tremendous(msg);
}
}
public class MonitorEngine
{
public static void principal(String[] args)
{
attempt
{
monitor();
}
catch (Exception e)
{
if (e instanceof PressureException)
System.out.println("correcting stress drawback");
else
System.out.println("correcting temperature drawback");
}
}
static void monitor() throws Exception
{
attempt
{
if (Math.random() < 0.1)
throw new PressureException("stress too excessive");
else
if (Math.random() > 0.9)
throw new TemperatureException("temperature too excessive");
else
System.out.println("all is properly");
}
catch (Exception e)
{
System.out.println(e.getMessage());
throw e;
}
}
}Itemizing 12 simulates the testing of an experimental rocket engine to see if the engine’s stress or temperature exceeds a security threshold. It performs this testing by way of the monitor() helper technique.
monitor()‘s attempt block throws PressureException when it detects a stress excessive, and throws TemperatureException when it detects a temperature excessive. (As a result of that is solely a simulation, random numbers are used — the java.lang.Math class’s static double random() technique returns a random quantity between 0.0 and (nearly) 1.0.) The attempt block is adopted by a catch block designed to partially deal with the exception by outputting a warning message. This exception is then re-thrown in order that monitor()‘s calling technique can end dealing with the exception.
Earlier than JDK 7 you couldn’t specify PressureException and TemperatureException in monitor()‘s throws clause as a result of the catch block’s e parameter is of kind java.lang.Exception and re-throwing an exception was handled as throwing the parameter’s kind. JDK 7 and successor JDKs have made it doable to specify these exception varieties within the throws clause as a result of their compilers can decide that the exception thrown by throw e comes from the attempt block, and solely PressureException and TemperatureException could be thrown from this block.
As a result of now you can specify static void monitor() throws PressureException, TemperatureException, you’ll be able to present extra exact handlers the place monitor() is named, as the next code fragment demonstrates:
attempt
{
monitor();
}
catch (PressureException pe)
{
System.out.println("correcting stress drawback");
}
catch (TemperatureException te)
{
System.out.println("correcting temperature drawback");
}Due to the improved kind checking provided by ultimate re-throw in JDK 7, supply code that compiled underneath earlier variations of Java would possibly fail to compile underneath later JDKs. For instance, think about Itemizing 13.
Itemizing 13. BreakageDemo.java
class SuperException extends Exception
{
}
class SubException1 extends SuperException
{
}
class SubException2 extends SuperException
{
}
public class BreakageDemo
{
public static void principal(String[] args) throws SuperException
{
attempt
{
throw new SubException1();
}
catch (SuperException se)
{
attempt
{
throw se;
}
catch (SubException2 se2)
{
}
}
}
}Itemizing 13 compiles underneath JDK 6 and earlier. Nevertheless, it fails to compile underneath later JDKs, whose compilers detect and report the truth that SubException2 isn’t thrown within the physique of the corresponding attempt assertion. This can be a small drawback that you’re unlikely to come across in your applications, and a worthwhile trade-off for having the compiler detect a supply of redundant code. Eradicating redundancies leads to cleaner code and smaller classfiles.
StackWalker and the StackWalking API
Acquiring a stack hint by way of Thread‘s or Throwable‘s getStackTrace() technique is expensive and impacts efficiency. The JVM eagerly captures a snapshot of your entire stack (aside from hidden stack frames), even if you solely want the primary few frames. Additionally, your code will most likely need to course of frames which are of no curiosity, which can be time-consuming. Lastly, you can’t entry the precise java.lang.Class occasion of the category that declared the strategy represented by a stack body. To entry this Class object, you’re pressured to increase java.lang.SecurityManager to entry the protected getClassContext() technique, which returns the present execution stack as an array of Class objects.
JDK 9 launched the java.lang.StackWalker class (with its nested Possibility class and StackFrame interface) as a extra performant and succesful different to StackTraceElement (plus SecurityManager). To find out about StackWalker and its associated varieties, see my introduction to the StackWalking API.
In conclusion
This text completes my two-part introduction to Java’s exception dealing with framework. You would possibly need to reinforce your understanding of this framework by reviewing Oracle’s Exceptions lesson within the Java Tutorials. One other good useful resource is Baeldung’s Exception dealing with in Java tutorial, which incorporates anti-patterns in exception dealing with.