A web service returns a huge XML and I need to access deeply nested fields of it. For example:
return wsObject.getFoo().getBar().getBaz().getInt()
The problem is that getFoo(), getBar(), getBaz() may all return null.
However, if I check for null in all cases, the code becomes very verbose and hard to read. Moreover, I may miss the checks for some of the fields.
if (wsObject.getFoo() == null) return -1;
if (wsObject.getFoo().getBar() == null) return -1;
// maybe also do something with wsObject.getFoo().getBar()
if (wsObject.getFoo().getBar().getBaz() == null) return -1;
return wsObject.getFoo().getBar().getBaz().getInt();
Is it acceptable to write
try {
return wsObject.getFoo().getBar().getBaz().getInt();
} catch (NullPointerException ignored) {
return -1;
}
or would that be considered an antipattern?
Catching NullPointerException is a really problematic thing to do since they can happen almost anywhere. It's very easy to get one from a bug, catch it by accident and continue as if everything is normal, thus hiding a real problem. It's so tricky to deal with so it's best to avoid altogether. (For example, think about auto-unboxing of a null Integer.)
I suggest that you use the Optional class instead. This is often the best approach when you want to work with values that are either present or absent.
Using that you could write your code like this:
public Optional<Integer> m(Ws wsObject) {
return Optional.ofNullable(wsObject.getFoo()) // Here you get Optional.empty() if the Foo is null
.map(f -> f.getBar()) // Here you transform the optional or get empty if the Bar is null
.map(b -> b.getBaz())
.map(b -> b.getInt());
// Add this if you want to return null instead of an empty optional if any is null
// .orElse(null);
// Or this if you want to throw an exception instead
// .orElseThrow(SomeApplicationException::new);
}
Why optional?
Using Optionals instead of null for values that might be absent makes that fact very visible and clear to readers, and the type system will make sure you don't accidentally forget about it.
You also get access to methods for working with such values more conveniently, like map and orElse.
Is absence valid or error?
But also think about if it is a valid result for the intermediate methods to return null or if that is a sign of an error. If it is always an error then it's probably better throw an exception than to return a special value, or for the intermediate methods themselves to throw an exception.
Maybe more optionals?
If on the other hand absent values from the intermediate methods are valid, maybe you can switch to Optionals for them also?
Then you could use them like this:
public Optional<Integer> mo(Ws wsObject) {
return wsObject.getFoo()
.flatMap(f -> f.getBar())
.flatMap(b -> b.getBaz())
.flatMap(b -> b.getInt());
}
Why not optional?
The only reason I can think of for not using Optional is if this is in a really performance critical part of the code, and if garbage collection overhead turns out to be a problem. This is because a few Optional objects are allocated each time the code is executed, and the VM might not be able to optimize those away. In that case your original if-tests might be better.
I suggest considering Objects.requireNonNull(T obj, String message). You might build chains with a detailed message for each exception, like
requireNonNull(requireNonNull(requireNonNull(
wsObject, "wsObject is null")
.getFoo(), "getFoo() is null")
.getBar(), "getBar() is null");
I would suggest you not to use special return-values, like -1. That's not a Java style. Java has designed the mechanism of exceptions to avoid this old-fashioned way which came from the C language.
Throwing NullPointerException is not the best option too. You could provide your own exception (making it checked to guarantee that it will be handled by a user or unchecked to process it in an easier way) or use a specific exception from XML parser you are using.
Assuming the class structure is indeed out of our control, as seems to be the case, I think catching the NPE as suggested in the question is indeed a reasonable solution, unless performance is a major concern. One small improvement might be to wrap the throw/catch logic to avoid clutter:
static <T> T get(Supplier<T> supplier, T defaultValue) {
try {
return supplier.get();
} catch (NullPointerException e) {
return defaultValue;
}
}
Now you can simply do:
return get(() -> wsObject.getFoo().getBar().getBaz().getInt(), -1);
As already pointed out by Tom in the comment,
Following statement disobeys the Law of Demeter,
wsObject.getFoo().getBar().getBaz().getInt()
What you want is int and you can get it from Foo. Law of Demeter says, never talk to the strangers. For your case you can hide the actual implementation under the hood of Foo and Bar.
Now, you can create method in Foo to fetch int from Baz. Ultimately, Foo will have Bar and in Bar we can access Int without exposing Baz directly to Foo. So, null checks are probably divided to different classes and only required attributes will be shared among the classes.
My answer goes almost in the same line as #janki, but I would like to modify the code snippet slightly as below:
if (wsObject.getFoo() != null && wsObject.getFoo().getBar() != null && wsObject.getFoo().getBar().getBaz() != null)
return wsObject.getFoo().getBar().getBaz().getInt();
else
return something or throw exception;
You can add a null check for wsObject as well, if there's any chance of that object being null.
You say that some methods "may return null" but do not say in what circumstances they return null. You say you catch the NullPointerException but you do not say why you catch it. This lack of information suggests you do not have a clear understanding of what exceptions are for and why they are superior to the alternative.
Consider a class method that is meant to perform an action, but the method can not guarantee it will perform the action, because of circumstances beyond its control (which is in fact the case for all methods in Java). We call that method and it returns. The code that calls that method needs to know whether it was successful. How can it know? How can it be structured to cope with the two possibilities, of success or failure?
Using exceptions, we can write methods that have success as a post condition. If the method returns, it was successful. If it throws an exception, it had failed. This is a big win for clarity. We can write code that clearly processes the normal, success case, and move all the error handling code into catch clauses. It often transpires that the details of how or why a method was unsuccessful are not important to the caller, so the same catch clause can be used for handling several types of failure. And it often happens that a method does not need to catch exceptions at all, but can just allow them to propagate to its caller. Exceptions due to program bugs are in that latter class; few methods can react appropriately when there is a bug.
So, those methods that return null.
Does a null value indicate a bug in your code? If it does, you should not be catching the exception at all. And your code should not be trying to second guess itself. Just write what is clear and concise on the assumption that it will work. Is a chain of method calls clear and concise? Then just use them.
Does a null value indicate invalid input to your program? If it does, a NullPointerException is not an appropriate exception to throw, because conventionally it is reserved for indicating bugs. You probably want to throw a custom exception derived from IllegalArgumentException (if you want an unchecked exception) or IOException (if you want a checked exception). Is your program required to provide detailed syntax error messages when there is invalid input? If so, checking each method for a null return value then throwing an appropriate diagnostic exception is the only thing you can do. If your program need not provide detailed diagnostics, chaining the method calls together, catching any NullPointerException and then throwing your custom exception is clearest and most concise.
One of the answers claims that the chained method calls violate the Law of Demeter and thus are bad. That claim is mistaken.
When it comes to program design, there are not really any absolute rules about what is good and what is bad. There are only heuristics: rules that are right much (even almost all) of the time. Part of the skill of programming is knowing when it is OK to break those kinds of rules. So a terse assertion that "this is against rule X" is not really an answer at all. Is this one of the situations where the rule should be broken?
The Law of Demeter is really a rule about API or class interface design. When designing classes, it is useful to have a hierarchy of abstractions. You have low level classes that uses the language primitives to directly perform operations and represent objects in an abstraction that is higher level than the language primitives. You have medium level classes that delegate to the low level classes, and implement operations and representations at a higher level than the low level classes. You have high level classes that delegate to the medium level classes, and implement still higher level operations and abstractions. (I've talked about just three levels of abstraction here, but more are possible). This allows your code to express itself in terms of appropriate abstractions at each level, thereby hiding complexity. The rationale for the Law of Demeter is that if you have a chain of method calls, that suggests you have a high level class reaching in through a medium level class to deal directly with low level details, and therefore that your medium level class has not provided a medium-level abstract operation that the high level class needs. But it seems that is not the situation you have here: you did not design the classes in the chain of method calls, they are the result of some auto-generated XML serialization code (right?), and the chain of calls is not descending through an abstraction hierarchy because the des-serialized XML is all at the same level of the abstraction hierarchy (right?)?
As others have said, respecting the Law of Demeter is definitely part of the solution. Another part, wherever possible, is to change those chained methods so they cannot return null. You can avoid returning null by instead returning an empty String, an empty Collection, or some other dummy object that means or does whatever the caller would do with null.
To improve readability, you may want to use multiple variables, like
Foo theFoo;
Bar theBar;
Baz theBaz;
theFoo = wsObject.getFoo();
if ( theFoo == null ) {
// Exit.
}
theBar = theFoo.getBar();
if ( theBar == null ) {
// Exit.
}
theBaz = theBar.getBaz();
if ( theBaz == null ) {
// Exit.
}
return theBaz.getInt();
Don't catch NullPointerException. You don't know where it is coming from (I know it is not probable in your case but maybe something else threw it) and it is slow.
You want to access the specified field and for this every other field has to be not null. This is a perfect valid reason to check every field. I would probably check it in one if and then create a method for readability. As others pointed out already returning -1 is very oldschool but I don't know if you have a reason for it or not (e.g. talking to another system).
public int callService() {
...
if(isValid(wsObject)){
return wsObject.getFoo().getBar().getBaz().getInt();
}
return -1;
}
public boolean isValid(WsObject wsObject) {
if(wsObject.getFoo() != null &&
wsObject.getFoo().getBar() != null &&
wsObject.getFoo().getBar().getBaz() != null) {
return true;
}
return false;
}
Edit: It is debatable if it's disobeyes the Law Of Demeter since the WsObject is probably only a data structure (check https://stackoverflow.com/a/26021695/1528880).
If you don't want to refactor the code and you can use Java 8, it is possible to use Method references.
A simple demo first (excuse the static inner classes)
public class JavaApplication14
{
static class Baz
{
private final int _int;
public Baz(int value){ _int = value; }
public int getInt(){ return _int; }
}
static class Bar
{
private final Baz _baz;
public Bar(Baz baz){ _baz = baz; }
public Baz getBar(){ return _baz; }
}
static class Foo
{
private final Bar _bar;
public Foo(Bar bar){ _bar = bar; }
public Bar getBar(){ return _bar; }
}
static class WSObject
{
private final Foo _foo;
public WSObject(Foo foo){ _foo = foo; }
public Foo getFoo(){ return _foo; }
}
interface Getter<T, R>
{
R get(T value);
}
static class GetterResult<R>
{
public R result;
public int lastIndex;
}
/**
* #param args the command line arguments
*/
public static void main(String[] args)
{
WSObject wsObject = new WSObject(new Foo(new Bar(new Baz(241))));
WSObject wsObjectNull = new WSObject(new Foo(null));
GetterResult<Integer> intResult
= getterChain(wsObject, WSObject::getFoo, Foo::getBar, Bar::getBar, Baz::getInt);
GetterResult<Integer> intResult2
= getterChain(wsObjectNull, WSObject::getFoo, Foo::getBar, Bar::getBar, Baz::getInt);
System.out.println(intResult.result);
System.out.println(intResult.lastIndex);
System.out.println();
System.out.println(intResult2.result);
System.out.println(intResult2.lastIndex);
// TODO code application logic here
}
public static <R, V1, V2, V3, V4> GetterResult<R>
getterChain(V1 value, Getter<V1, V2> g1, Getter<V2, V3> g2, Getter<V3, V4> g3, Getter<V4, R> g4)
{
GetterResult result = new GetterResult<>();
Object tmp = value;
if (tmp == null)
return result;
tmp = g1.get((V1)tmp);
result.lastIndex++;
if (tmp == null)
return result;
tmp = g2.get((V2)tmp);
result.lastIndex++;
if (tmp == null)
return result;
tmp = g3.get((V3)tmp);
result.lastIndex++;
if (tmp == null)
return result;
tmp = g4.get((V4)tmp);
result.lastIndex++;
result.result = (R)tmp;
return result;
}
}
Output
241
4
null
2
The interface Getter is just a functional interface, you may use any equivalent.
GetterResult class, accessors stripped out for clarity, hold the result of the getter chain, if any, or the index of the last getter called.
The method getterChain is a simple, boilerplate piece of code, that can be generated automatically (or manually when needed).
I structured the code so that the repeating block is self evident.
This is not a perfect solution as you still need to define one overload of getterChain per number of getters.
I would refactor the code instead, but if can't and you find your self using long getter chains often you may consider building a class with the overloads that take from 2 to, say, 10, getters.
I'd like to add an answer which focus on the meaning of the error. Null exception in itself doesn't provide any meaning full error. So I'd advise to avoid dealing with them directly.
There is a thousands cases where your code can go wrong: cannot connect to database, IO Exception, Network error... If you deal with them one by one (like the null check here), it would be too much of a hassle.
In the code:
wsObject.getFoo().getBar().getBaz().getInt();
Even when you know which field is null, you have no idea about what goes wrong. Maybe Bar is null, but is it expected? Or is it a data error? Think about people who read your code
Like in xenteros's answer, I'd propose using custom unchecked exception. For example, in this situation: Foo can be null (valid data), but Bar and Baz should never be null (invalid data)
The code can be re-written:
void myFunction()
{
try
{
if (wsObject.getFoo() == null)
{
throw new FooNotExistException();
}
return wsObject.getFoo().getBar().getBaz().getInt();
}
catch (Exception ex)
{
log.error(ex.Message, ex); // Write log to track whatever exception happening
throw new OperationFailedException("The requested operation failed")
}
}
void Main()
{
try
{
myFunction();
}
catch(FooNotExistException)
{
// Show error: "Your foo does not exist, please check"
}
catch(OperationFailedException)
{
// Show error: "Operation failed, please contact our support"
}
}
NullPointerException is a run-time exception, so generally speaking is not recommended to catch it, but to avoid it.
You will have to catch the exception wherever you want to call the method (or it will propagate up the stack). Nevertheless, if in your case you can keep working with that result with value -1 and you are sure that it won't propagate because you are not using any of the "pieces" that may be null, then it seems right to me to catch it
Edit:
I agree with the later answer from #xenteros, it wil be better to launch your own exception instead returning -1 you can call it InvalidXMLException for instance.
Have been following this post since yesterday.
I have been commenting/voting the comments which says, catching NPE is bad. Here is why I have been doing that.
package com.todelete;
public class Test {
public static void main(String[] args) {
Address address = new Address();
address.setSomeCrap(null);
Person person = new Person();
person.setAddress(address);
long startTime = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
try {
System.out.println(person.getAddress().getSomeCrap().getCrap());
} catch (NullPointerException npe) {
}
}
long endTime = System.currentTimeMillis();
System.out.println((endTime - startTime) / 1000F);
long startTime1 = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
if (person != null) {
Address address1 = person.getAddress();
if (address1 != null) {
SomeCrap someCrap2 = address1.getSomeCrap();
if (someCrap2 != null) {
System.out.println(someCrap2.getCrap());
}
}
}
}
long endTime1 = System.currentTimeMillis();
System.out.println((endTime1 - startTime1) / 1000F);
}
}
public class Person {
private Address address;
public Address getAddress() {
return address;
}
public void setAddress(Address address) {
this.address = address;
}
}
package com.todelete;
public class Address {
private SomeCrap someCrap;
public SomeCrap getSomeCrap() {
return someCrap;
}
public void setSomeCrap(SomeCrap someCrap) {
this.someCrap = someCrap;
}
}
package com.todelete;
public class SomeCrap {
private String crap;
public String getCrap() {
return crap;
}
public void setCrap(String crap) {
this.crap = crap;
}
}
Output
3.216
0.002
I see a clear winner here. Having if checks is way too less expensive than catch an exception. I have seen that Java-8 way of doing. Considering that 70% of the current applications still run on Java-7 I am adding this answer.
Bottom Line For any mission critical applications, handling NPE is costly.
If efficiency is an issue then the 'catch' option should be considered.
If 'catch' cannot be used because it would propagate (as mentioned by 'SCouto') then use local variables to avoid multiple calls to methods getFoo(), getBar() and getBaz().
It's worth considering to create your own Exception. Let's call it MyOperationFailedException. You can throw it instead returning a value. The result will be the same - you'll quit the function, but you won't return hard-coded value -1 which is Java anti-pattern. In Java we use Exceptions.
try {
return wsObject.getFoo().getBar().getBaz().getInt();
} catch (NullPointerException ignored) {
throw new MyOperationFailedException();
}
EDIT:
According to the discussion in comments let me add something to my previous thoughts. In this code there are two possibilities. One is that you accept null and the other one is, that it is an error.
If it's an error and it occurs, You can debug your code using other structures for debugging purposes when breakpoints aren't enough.
If it's acceptable, you don't care about where this null appeared. If you do, you definitely shouldn't chain those requests.
The method you have is lengthy, but very readable. If I were a new developer coming to your code base I could see what you were doing fairly quickly. Most of the other answers (including catching the exception) don't seem to be making things more readable and some are making it less readable in my opinion.
Given that you likely don't have control over the generated source and assuming you truly just need to access a few deeply nested fields here and there then I would recommend wrapping each deeply nested access with a method.
private int getFooBarBazInt() {
if (wsObject.getFoo() == null) return -1;
if (wsObject.getFoo().getBar() == null) return -1;
if (wsObject.getFoo().getBar().getBaz() == null) return -1;
return wsObject.getFoo().getBar().getBaz().getInt();
}
If you find yourself writing a lot of these methods or if you find yourself tempted to make these public static methods then I would create a separate object model, nested how you would like, with only the fields you care about, and convert from the web services object model to your object model.
When you are communicating with a remote web service it is very typical to have a "remote domain" and an "application domain" and switch between the two. The remote domain is often limited by the web protocol (for example, you can't send helper methods back and forth in a pure RESTful service and deeply nested object models are common to avoid multiple API calls) and so not ideal for direct use in your client.
For example:
public static class MyFoo {
private int barBazInt;
public MyFoo(Foo foo) {
this.barBazInt = parseBarBazInt();
}
public int getBarBazInt() {
return barBazInt;
}
private int parseFooBarBazInt(Foo foo) {
if (foo() == null) return -1;
if (foo().getBar() == null) return -1;
if (foo().getBar().getBaz() == null) return -1;
return foo().getBar().getBaz().getInt();
}
}
return wsObject.getFooBarBazInt();
by applying the the Law of Demeter,
class WsObject
{
FooObject foo;
..
Integer getFooBarBazInt()
{
if(foo != null) return foo.getBarBazInt();
else return null;
}
}
class FooObject
{
BarObject bar;
..
Integer getBarBazInt()
{
if(bar != null) return bar.getBazInt();
else return null;
}
}
class BarObject
{
BazObject baz;
..
Integer getBazInt()
{
if(baz != null) return baz.getInt();
else return null;
}
}
class BazObject
{
Integer myInt;
..
Integer getInt()
{
return myInt;
}
}
Giving answer which seems different from all others.
I recommend you to check for NULL in ifs.
Reason :
We should not leave a single chance for our program to be crashed.
NullPointer is generated by system. The behaviour of System
generated exceptions can not be predicted. You should not leave your
program in the hands of System when you already have a way of handling
it by your own. And put the Exception handling mechanism for the extra safety.!!
For making your code easy to read try this for checking the conditions :
if (wsObject.getFoo() == null || wsObject.getFoo().getBar() == null || wsObject.getFoo().getBar().getBaz() == null)
return -1;
else
return wsObject.getFoo().getBar().getBaz().getInt();
EDIT :
Here you need to store these values wsObject.getFoo(),
wsObject.getFoo().getBar(), wsObject.getFoo().getBar().getBaz() in
some variables. I am not doing it because i don't know the return
types of that functions.
Any suggestions will be appreciated..!!
I wrote a class called Snag which lets you define a path to navigate through a tree of objects. Here is an example of its use:
Snag<Car, String> ENGINE_NAME = Snag.createForAndReturn(Car.class, String.class).toGet("engine.name").andReturnNullIfMissing();
Meaning that the instance ENGINE_NAME would effectively call Car?.getEngine()?.getName() on the instance passed to it, and return null if any reference returned null:
final String name = ENGINE_NAME.get(firstCar);
It's not published on Maven but if anyone finds this useful it's here (with no warranty of course!)
It's a bit basic but it seems to do the job. Obviously it's more obsolete with more recent versions of Java and other JVM languages that support safe navigation or Optional.
For better debugging, I would often like to have:
Exception
at com.example.blah.Something.method()
at com.example.blah.Xyz.otherMethod()
at com.example.hello.World.foo()
at com.example.debug.version_3_8_0.debug_info_something.Hah.method() // synthetic method
at com.example.x.A.wrappingMethod()
The debug stack frame as shown above would be dynamically generated, just like a java.lang.reflect.Proxy, except that I'd like to be in full control of the entire fully qualified method name that ends up on the proxy.
At the call site, I would do something silly and simple as this:
public void wrappingMethod() {
run("com.example.debug.version_3_8_0.debug_info_something.Hah.method()", () -> {
World.foo();
});
}
As you can see, the wrappingMethod() is a real method that ends up on the stack trace, Hah.method() is a dynamically generated method, whereas World.foo() is again a real method.
Yes, I know this pollutes the already deep deep stack traces. Don't worry about it. I have my reasons.
Is there a (simple) way to do this or something similar as the above?
No need for code generation to solve this problem:
static void run(String name, Runnable runnable) {
try {
runnable.run();
} catch (Throwable throwable) {
StackTraceElement[] stackTraceElements = throwable.getStackTrace();
StackTraceElement[] currentStackTrace = new Throwable().getStackTrace();
if (stackTraceElements != null && currentStackTrace != null) { // if disabled
int currentStackSize = currentStackStrace.length;
int currentFrame = stackTraceElements.length - currentStackSize - 1;
int methodIndex = name.lastIndexOf('.');
int argumentIndex = name.indexOf('(');
stackTraceElements[currentFrame] = new StackTraceElement(
name.substring(0, methodIndex),
name.substring(methodIndex + 1, argumentIndex),
null, // file name is optional
-1); // line number is optional
throwable.setStackTrace(stackTraceElements);
}
throw throwable;
}
}
With code generation, you could add a method with the name, redefine the call site within the method, unwind the frame and call the generated method but this would be much more work and would never be equally stable.
This strategy is a rather common approach in testing frameworks, we do it a lot in Mockito and also other utilities like JRebel do it to hide their magic by rewriting exception stack frames.
When Java 9 is used, it would be more efficient to do such manipulations using the Stack Walker API.
This is a client-server programm.
For each client server has a method, which checks if there are some messages to this client.
Code:
while (bool) {
for(int j = 0;j<Start.bases.size();j++){
if(Start.bases.get(j).getId() == id){
if(!Start.bases.get(j).ifEmpty()){
String output = Start.bases.get(j).getMessage();
os.println(output);
System.out.println(output +" *FOT* "+ addr.getHostName());
}
}
}
Each thread has an id.
So everything seems to be OK, but I get strange null pointer Exception at this line
if(Start.bases.get(j).getId() == id){
id - integer.
It is really strange, because I have run in debug this part and checked that "bases" and "id" are not null and bases have apropriate fields.
bases is not empty.
By the way bases is static(because every thread can use it) and bases is declared before this method is used.
This line doesn't cause problems
for(int j = 0;j<Start.bases.size();j++){
May it is because of method getId() ?
public int getId(){
return id;
}
What is the problem?
Edited.
static ArrayList<Base> bases;
bases = new ArrayList<Base>();
Class Base:
public class Base {
private ServerThread st;
private int id;
private String name;
private ArrayList<String> messages;
public Base(String n, ServerThread s_t, int i_d){
messages = new ArrayList<String>();
st = s_t;
name = n;
id = i_d;
}
public String getName(){
return name;
}
public int getId(){
return id;
}
public ServerThread getThr(){
return st;
}
public String getMessage(){
String ret = "";
if(!messages.isEmpty()){
ret = messages.get(0);
messages.remove(messages.get(0));
}
return ret;
}
public void addMessage(String m){
messages.add(m);
}
public boolean ifEmpty(){
return messages.isEmpty();
}
}
Thanks.
In this line of code:
(Start.bases.get(j).getId() == id
you may have such exception in such cases:
1) bases is null - you said its wrong
2) bases.get(j) - it may occur only if you collection size was reduced during iteration(as mentioned Gray)
3) Start.bases.get(j).getId() is null. But as you mentioned getId() method return primitive int, so its not the case as in this situation you receive null ponter while casting - in line " return id;".
So you should check second case.
Given this:
"I have run in debug this part and checked that "bases" and "id" are not null and bases have apropriate fields"
and this:
bases is static(because every thread can use it)
I think it's pretty likely that you have a race condition. In a race condition, there are two threads simultaneously accessing the same data structure (in this case, Start.bases). Most of the time, one thread's code completes faster, and everything goes the way you expect them to, but occasionally the other thread gets a head-start or goes a little faster than usual and things go "boom".
When you introduce a debugger with a break point, you pretty much guarantee that the code with the break point will execute last, because you've stopped it mid-execution while all your other threads are still going.
I'd suggest that the size of your list is probably changing as you execute. When a user leaves, is their entry removed from the "base" list? Is there some other circumstance where the list can be changed from another thread during execution?
The first thing I'll suggest is that you switch your code to use iterators rather than straight "for" loops. It won't make the problem go away (it might actually make it more visible), but it will make what's happening a lot clearer. You'll get a ConcurrentModificationException at the point where the modification happens, rather than the less helpful NullPointerException only when a certain combination of changes happens.):
for(Base currentBase : Start.bases)
{
if(currentBase.getId() == id && !currentBase.ifEmpty())
{
String output = currentBase.getMessage();
os.println(output);
System.out.println(output +" *FOT* "+ addr.getHostName());
}
}
If you do get a concurrent modification exception with the above code, then you're definitely dealing with a race condition. That means that you'll have to synchronize your code.
There are a couple of ways to do this, depending on how your application is structured.
Assuming that the race is only between this bit of code and one other (the part doing the removing-from-the-list), you can probably solve this scenario by wrapping both chunks of code in
synchronized(Start.bases)
{
[your for-loop/item removal code goes here]
}
This will acquire a lock on the list itself, so that those two pieces of code will not attempt to update the same list at the same time in different threads. (Note that it won't stop concurrent modification to the Base objects themselves, but I doubt that's the problem in this case).
All of that said, any time you have a variable which is read/write accessed by multiple threads it really should be synchronized. That's a fairly complicated job. It's better to keep the synchronization inside the object you're managing if you can. That way you can see all the synchronization code in one place, making you less likely to accidentally create deadlocks. (In your code above, you'd need to make the "for" loop a method inside your Start class, along with anything else which uses that list, then make "bases" private so that the rest of the application must use those methods).
Without seeing all the other places in your code where this list is accessed, I can't say exactly what changes you should make, but hopefully that's enough to get you started. Remember that multi-threading in Java requires a very delicate hand!
I need to be able to re-use a java.io.InputStream multiple times, and I figured the following code would work, but it only works the first time.
Code
public class Clazz
{
private java.io.InputStream dbInputStream, firstDBInputStream;
private ArrayTable db;
public Clazz(java.io.InputStream defDB)
{
this.firstDBInputStream = defDB;
this.dbInputStream = defDB;
if (db == null)
throw new java.io.FileNotFoundException("Could not find the database at " + db);
if (dbInputStream.markSupported())
dbInputStream.mark(Integer.MAX_VALUE);
loadDatabaseToArrayTable();
}
public final void loadDatabaseToArrayTable() throws java.io.IOException
{
this.dbInputStream = firstDBInputStream;
if (dbInputStream.markSupported())
dbInputStream.reset();
java.util.Scanner fileScanner = new java.util.Scanner(dbInputStream);
String CSV = "";
for (int i = 0; fileScanner.hasNextLine(); i++)
CSV += fileScanner.nextLine() + "\n";
db = ArrayTable.createArrayTableFromCSV(CSV);
}
public void reloadDatabase()//A method called by the UI
{
try
{
loadDatabaseToArrayTable();
}
catch (Throwable t)
{
//Alert the user that an error has occurred
}
}
}
Note that ArrayTable is a class of mine, which uses arrays to give an interface for working with tables.
Question
In this program, the database is shown directly to the user immediately after the reloadDatabase() method is called, and so any solution involving saving the initial read to an object in memory is useless, as that will NOT refresh the data (think of it like a browser; when you press "Refresh", you want it to fetch the information again, not just display the information it fetched the first time). How can I read a java.io.InputStream more than once?
You can't necessarily read an InputStream more than once. Some implementations support it, some don't. What you are doing is checking the markSupported method, which is indeed an indicator if you can read the same stream twice, but then you are ignoring the result. You have to call that method to see if you can read the stream twice, and if you can't, make other arrangements.
Edit (in response to comment): When I wrote my answer, my "other arrangements" was to get a fresh InputStream. However, when I read in your comments to your question about what you want to do, I'm not sure it is possible. For the basics of the operation, you probably want RandomAccessFile (at least that would be my first guess, and if it worked, that would be the easiest) - however you will have file access issues. You have an application actively writing to a file, and another reading that file, you will have problems - exactly which problems will depend on the OS, so whatever solution would require more testing. I suggest a separate question on SO that hits on that point, and someone who has tried that out can perhaps give you more insight.
you never mark the stream to be reset
public Clazz(java.io.InputStream defDB)
{
firstDBInputStream = defDB.markSupported()?defDB:new BufferedInputStream(defDB);
//BufferedInputStream supports marking
firstDBInputStream.mark(500000);//avoid IOException on first reset
}
public final void loadDatabaseToArrayTable() throws java.io.IOException
{
this.dbInputStream = firstDBInputStream;
dbInputStream.reset();
dbInputStream.mark(500000);//or however long the data is
java.util.Scanner fileScanner = new java.util.Scanner(dbInputStream);
StringBuilder CSV = "";//StringBuilder is more efficient in a loop
while(fileScanner.hasNextLine())
CSV.append(fileScanner.nextLine()).append("\n");
db = ArrayTable.createArrayTableFromCSV(CSV.toString());
}
however you could instead keep a copy of the original ArrayTable and copy that when you need to (or even the created string to rebuild it)
this code creates the string and caches it so you can safely discard the inputstreams and just use readCSV to build the ArrayTable
private String readCSV=null;
public final void loadDatabaseToArrayTable() throws java.io.IOException
{
if(readCSV==null){
this.dbInputStream = firstDBInputStream;
java.util.Scanner fileScanner = new java.util.Scanner(dbInputStream);
StringBuilder CSV = "";//StringBuilder is more efficient in a loop
while(fileScanner.hasNextLine())
CSV.append(fileScanner.nextLine()).append("\n");
readCSV=CSV.toString();
fileScanner.close();
}
db = ArrayTable.createArrayTableFromCSV(readCSV);
}
however if you want new information you'll need to create a new stream to read from again