How to avoid many try catch blocks in java

I'm very new to java and the idea of try catch blocks to handle exceptions.

This roughly what I'm ending up with, and there simply has to be a better way:

    try {
 开发者_如何学Python       JSONObject jsonObject = new JSONObject(jsonString);
        int aCount = jsonObject.getInt("acount");
        String devTok = jsonObject.getString("dt");
        String qURL = jsonObject.getString("qu");
        try {
            DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
            Key qKey = KeyFactory.createKey("qu", qURL);
            int dsACount = (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
            //..etc.. more try catch blocks needed
        } catch (EntityNotFoundException e) {
            e.printStackTrace();
        }
    } catch (com.google.appengine.repackaged.org.json.JSONException e) {
        e.printStackTrace();
    }

There are more try catch blocks embedded in the same way, so that at the end, there is just a lump of catch blocks. How else should exceptions be handled, Eclipse keeps asking my to either surround with a try catch block or "Add throws declaration".

Sometimes I want to catch certain exceptions, for example if it can't find an entity, I want to print something like "Entity not found", and if the JSON string can't be parsed into an object, I want to print something like "Can't parse JSON".

(I'm used to objective-c where the are delegate methods for failure, or the method returns null and you have passed a pointer to [a pointer to] an NSError object, which will have been "filled", Is there somewhere to learn about try-catch?)

If all you're doing is catching them and printing the stack trace regardless of the exception type, you can just wrap the code in one large try/catch block. To save many "catches", you can catch java.lang.Throwable which is the interface that all exceptions implement. If not, you can have a catch for every type of checked exception the code you're calling throws, and handle them specifically.

Eclipse keeps asking you to do so because Java code will not compile if the checked exceptions are not caught, or declared to be thrown by the caller.

+Adding this comment to the answer (Thanks, Paul Tomblin):

In production quality apps you'd be logging the trace, adding some logic where you're handling the exception in a right way, taking an alternate flow, and/or re-wrapping it in another exception and throwing it, etc. It all depends on the particular problem you're trying to solve.

The idea of exception handling is that you can handle errors at points in your program flow where you can deal with them meaningfully. Rather than checking every function's return value like in C, where most of the time you can't do anything sensible other than passing the error further up, you install a try/catch block at sensible points in your program:

Basically, whenever there is a point where you can react meaningfully to an error, then catch that error, and pass everything else on. That way, error handling is only invoked when there is a plausible recovery from the error.

For example, worst case if any error stops your program from executing meaningfully, then you might almost not catch anything at all and just let the OS handle the situation (well, perhaps one single try/catch to produce a friendly error message).

Example (in C++, sorry, I'm can't type Java blind):

int main()
{
  try {
    while (masterloop()) { }
  catch (...) {
    LOG("Fatal program error, terminating!"); // nothing else we can do!
  }
}

/* lots of program logic */

void process_image()
{
  try {
    Image im = load_image_from_disk();
    /* ... */
  }
  catch (const OutOfMemoryExc & e) {
    LOG("Not enough memory to process the image.");
    return;
  }
  catch (const DataErrorExc & e) {
    LOG("Could not read the image data.");
    return;
  }
  catch (...) {
    throw; // pass everything else along
  }
}

In this example, we may try to process an image and fail for some anticipable reasons (out of memory, or failure to read the image). In that case we just return without doing work and let the program continue gracefully. All other errors are propagated up to a higher point. Most importantly, we do not need to litter the actual image processing function with error checks and responses all the time, it suffices for any code there to throw one of our two good exceptions and not worry any further.

Moral: If you have try/catch blocks absolutely everywhere, you're doing it wrong.

I know there's a lot of answers here, and they do a good job of covering how to structure the try/catch blocks. However, I'm thinking one of the things bothering you is the significant... indentation and code growth (... because I know it's not the indentation or amount of code, but the implied complexity by wrapping it and shifting it over and growing longer and longer between the opening try and enclosing catch, and I can't put a word to that apprehension).

The way to get around this is to refactor into functions the distinct bits in the code. I know it's a simplistic answer, but it's a good way to isolate individual tasks and keep the error handling fairly local to the code that requires it without padding things out vertically and horizontally with nested try/catch blocks.

You can make these methods private as they are intended for internal use only, presumably.

private Integer getDatastoreACount() {
    try {
        DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
        Key qKey = KeyFactory.createKey("qu", qURL);
        return (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
        //..etc.. more try catch blocks needed
    } catch (EntityNotFoundException e) {
        // expects an Integer return, so need to deal with this
        // but for simplicity I'm just simply recycling 'e'
        throw e;
    }
}

public void parseJSON(String jsonString) {
    try {
        JSONObject jsonObject = new JSONObject(jsonString);
        int aCount = jsonObject.getInt("acount");
        String devTok = jsonObject.getString("dt");
        String qURL = jsonObject.getString("qu");
        Integer dsACount = getDatastoreACount();
        //etc etc
    } catch (com.google.appengine.repackaged.org.json.JSONException e) {
        e.printStackTrace();
    }
}

You can catch multiple exceptions in the same try e.g.

try{

  xyz;

}catch(NullPointerException npx){
  npx.getMessage();
}catch(ArrayOutOfBoundsException ax){
  ax.getMessage();
}

Also, by declaring the Exception as throws in your method signatures you can pass the Exception up the stack.

If you're just doing something like this:

try {
  do smth
  try {
    do smth more
    ...
  } catch (Exception1 e1) {reaction to e1}
} catch (Exception2 e2) {reaction to e2}

You can do everything in one try-block:

try {
  do smth
  do smth more
  ...
}
catch (Exception1 e1) {reaction to e1}
catch (Exception2 e2) {reaction to e2}

You can also break this down to one catch block if you're just printing the exception:

try {
  do smth
  do smth more
  ...
}
catch (Exception e) {e.printStackTrace();}

But this doesn't if you want to do somthing more, even if e1 is thrown, like:

try {
  do smth
  try {
    do smth more
    ...
  } catch (Exception1 e1) {reaction to e1}
  do smth even if e1 was thrown
} catch (Exception2 e2) {reaction to e2}

The last example can't be written shorter.

If you have a block of code in which more than one type of exception may be thrown, you can declare two separate catch blocks:

try {
    JSONObject jsonObject = new JSONObject(jsonString);
    int aCount = jsonObject.getInt("acount");
    String devTok = jsonObject.getString("dt");
    String qURL = jsonObject.getString("qu");

    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
    Key qKey = KeyFactory.createKey("qu", qURL);
    int dsACount = (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
} catch (EntityNotFoundException e) {
    e.printStackTrace();
} catch (com.google.appengine.repackaged.org.json.JSONException e) {
    e.printStackTrace();
}
//..etc.. as many catch blocks as needed

Alternatively, if you don't care about the exact type of the exception, you can have onyl one catch block which catches Exception (or maybe Throwable; I can't remember exactly what the superclass of exceptions is in Java).

Another point I will make now is that you might not have the most modular code. Remember, code that does one thing well makes for good, modular code. If you find that you have many nested black (whether try/catch blocks, if/else blocks, etc.) you may want to check if some of the code can be extracted into its own method. This may also make your code look better when many exceptions must be handled.

First, from a design perspective, catching and printing exceptions is not a good thing. Something went wrong, and your code just keeps going in the same way as if it went right. That is not usually correct. So: perhaps your method needs to throw these exceptions instead of catching them. Perhaps only the caller is in a position to decide what happens if something fails like this.

But otherwise, the only advice I can offer to clean up how the code looks, syntactically, is to tell you that you can write:

try {
  ...
} catch (...) {
  ...
} catch (...) {
  ...
}

You can also catch for a broader exception class like Exception and just write one catch block but this is bad design. In Java 7, you will be able to catch for several exception types in one block.

You should use try/catch blocks if you have a way to recover from the exception, for example if you want to check if a string is a valid integer, you can write a method (this is a lame method, but just to show the idea):

public boolean isInteger(String str) {
    try {
        new Integer(str);
    }
    catch(NumberFormatException e) {
        return false;
    }
    return true;
}

If you don't have a way to recover from the exception and all you do is to print the stack trace, it is suggested to add throws declaration (as eclipse suggest) to the method, and let the caller handle the exception (or throw it to its caller).

If you want to handle some exceptions and throw some other, you can do it as well.

I like to box up the call behind a static method, just to keep it tidier. For example, here's my reduced Set Json Value call.

private static boolean setJsonValue(JSONObject j,String key,Object value)
{
    try 
    {
        if(value instanceof Integer)
        {
            // numbers are special. We want them unquoted.
            int valueI = (Integer)value;
            j.put(key,valueI);
        }
        else
            j.put(key,value);
        return true;
    }
    catch (JSONException e) 
    {
        // do nothing, it turns out
        return false;
    }
}

...and then I ignore the return values, because I am bad.

Somewhere or other I have a similar Get method, that returns null if it fails. You get the idea.

You have two basic code-style choices here (that don't involve changing method signatures)

Method1: Put everything in the one try catch and have multiple catch blocks, like this:

try {
    JSONObject jsonObject = new JSONObject(jsonString);
    int aCount = jsonObject.getInt("acount");
    String devTok = jsonObject.getString("dt");
    String qURL = jsonObject.getString("qu");
    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
    Key qKey = KeyFactory.createKey("qu", qURL);
    int dsACount = (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
    //..etc.. more try catch blocks needed
} catch (EntityNotFoundException e) {
    e.printStackTrace();
} catch (com.google.appengine.repackaged.org.json.JSONException e) {
    e.printStackTrace();
}

Method 2: Break up your code into sections that each have one catch, like this:

String qURL = null;
try {
    JSONObject jsonObject = new JSONObject(jsonString);
    int aCount = jsonObject.getInt("acount");
    String devTok = jsonObject.getString("dt");
    String qURL = jsonObject.getString("qu");
} catch (EntityNotFoundException e) {
    e.printStackTrace();
} 

try {    
    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
    Key qKey = KeyFactory.createKey("qu", qURL);
    int dsACount = (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
} catch (EntityNotFoundException e) {
    e.printStackTrace();
} 

Method 2 is the recommended one, as it makes it obvious which lines are throwing which exceptions and generally segments the code into natural processing blocks.

Create another exception and put it below or above of the other exception. Depends on the context of your application.