I am trying an example from
http://www.roseindia.net/java/beginners/java-read-file-line-by-line.shtml
in the example the BufferReader is not closed is that necessary to close the BufferReaderor not? Please explain.
FileInputStream fstream = new FileInputStream("textfile.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(fstream));
String strLine;
//Read File Line By Line
while ((strLine = br.readLine()) != null) {
// Print the content on the console
System.out.println (strLine);
}
//Close the input stream
in.close();
Always close streams. It's a good habit which helps you to avoid some odd behaviour. Calling close() method also calls flush() so you don't have do this manually.
The best place where to close streams is probably in a finally block. If you have it like in your example and an exception occurs before the in.close() line, the stream won't be closed.
And if you have chained streams, you can only close the last one and all before it are closed too. This means br.close() in your example - not in.close();
Example
try {
// do something with streams
} catch (IOException e) {
// process exception - log, wrap into your runtime, whatever you want to...
} finally {
try {
stream.close();
} catch (IOException e) {
// error - log it at least
}
}
Alternatively you can use closeQuietly(java.io.InputStream) in Apache Commons library.
From the perspective of resource leak prevention, it is not strictly necessary to close a wrapper stream if you've also closed the stream that it wraps. However, closing the wrapped stream may result in stuff getting lost (specifically in the output case), so it is better to close (just) the wrapper, and rely on documented behavior that the closing the wrapper closes the wrapped stream too. (That is certainly true for the standard I/O wrapper classes!)
Like Peter Lawrey, I question the wisdom of relying on "Rose India" examples. For instance, this one has two more obvious mistakes in it that no half-decent Java programmer should make:
The stream is not closed in a finally block. If any exception is thrown between opening and closing, the in.close() statement won't be executed, and the application will leak an open file descriptor. Do that too often and your application will start throwing unexpected IOExceptions.
The DataInputStream in the chain serves no useful purpose. Instead, they should use fstream as the parameter for the InputStreamReader. Or better still, use FileReader.
Finally, here is a corrected version of the example:
BufferedReader br = new BufferedReader(new FileReader ("textfile.txt"));
try {
String line;
while ((line = br.readLine()) != null) {
// Print the content on the console
System.out.println(line);
}
} finally {
// Close the reader stack.
br.close();
}
or using Java 7's "try with resource":
try (BufferedReader br = new BufferedReader(new FileReader ("textfile.txt"))) {
String line;
while ((line = br.readLine()) != null) {
// Print the content on the console
System.out.println(line);
}
}
Since the underlying stream is closed, it is not absolutely necessary to close BufferedReader, even though it is a good practice to close ALL Closeables in reverse order (relative to the order they were opened in.)
Related
I am writing a method but see this error: may fail to close stream.
According to some solutions on different posts, I have added try and catch within the finally block. I also added IOUtils.closeQuietly(fullObject, (Log) LOGGER). But it still doesn't work. Anyone can help take a look? Thanks!
S3Object fullObject = null;
StringBuffer buffer = new StringBuffer();
try {
S3Object s3Response = s3Client.getObject(s3BucketName, s3Key);
BufferedReader reader = new BufferedReader(new InputStreamReader(s3Response.getObjectContent()));
String line;
while ((line = reader.readLine()) != null) {
buffer.append(line);
}
} finally {
if (fullObject != null) {
try {
fullObject.close();
} catch (IOException ex) {
throw new RuntimeException(ex);
}
IOUtils.closeQuietly(fullObject, (Log) LOGGER);
}
}
return buffer.toString();
}
You should be using Java 7+ try with resources. It will take care of closing the resources you declare in the list. Any exceptions that may be thrown in the process of closing will be dealt with appropriately. (They are either allowed to propagate, or they are "suppressed" if an exception was already propagating.)
Your code using try with resources would look like this. It is half the length of the original version AND it won't have any resource leaks. You "win" both ways.
try (S3Object s3Response = s3Client.getObject(s3BucketName, s3Key);
BufferedReader reader = new BufferedReader(
new InputStreamReader(s3Response.getObjectContent()));
)
{
StringBuffer buffer = new StringBuffer();
String line;
while ((line = reader.readLine()) != null) {
buffer.append(line);
}
return buffer.toString();
}
Notice that I have gotten rid of fullObject which your code wasn't using.
There are actually two managed resources in the above: the s3Response and the reader. It might not be strictly necessary to close both, but (IMO) closing them anyway is the correct thing to do ... from the perspective of readability, if nothing else.
(It may also be possible to do the "read content as a string" more simply and/or more efficiently, but that is outside of the scope of this question.)
InputStreamReader implements AutoCloseable. This means that the intended use is try-with-resources:
try (InputStreamReader reader = new InputStreamReader(s3Response.getObjectContent()) {
...
}
This should always close the stream irrespective of how the block exits (i.e. through normal completion, catch or finally clauses).
The same is true for S3Object and BufferedReader. They can all be declared as resources within the same try block.
See https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html for more details.
I'm writing code to read a file and process it and I'm splitting logic into many small methods. So I have a method to read the file and return BufferedReader and another one to do logic with the returned BufferedReader object. But when I try to read lines from the BufferedReader object in the second method it gives me [java.io.IOException: Stream Closed].
The method I used to read the file and return BufferedReader
private static BufferedReader readFile(String file) {
try (FileInputStream fileInputStream = new FileInputStream(file)) {
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(fileInputStream));
System.out.println(bufferedReader.readLine()); // this line is working successfully
return bufferedReader;
} catch (FileNotFoundException fileNotFoundException) {
fileNotFoundException.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
Any idea why the happens and how to solve it ?
You are using try-with-resources:
try (FileInputStream fileInputStream = new FileInputStream(file)) {
This line is creating a FileInputStream which can be used inside your try block. As soon as you leave your try block, the close() method will be called onto the stream. So if you return the stream or its BufferedReader, the stream will already be closed. You should not use try-with-resources or even better, return whatever you need from the stream instead of the stream itself.
Obviously, the problem here lies in the use of try-with-resources, and AutoCloseable interface. However, i would like to point out that the way you read the file is the "old fashioned" way. We now have the Files, Paths, and Stream classes to facilitate reading of files. this uses java NIO, returns a Stream and is better overall:
Files.lines(Paths.get(pathToFile))
additionally, nothing has to be closed here
I've got the following piece of code:
public ArrayList<Crime> loadCrimes() throws IOException, JSONException {
ArrayList<Crime> crimes = new ArrayList<Crime>();
BufferedReader reader = null;
try {
// Open and read the file into a StringBuilder
InputStream in = mContext.openFileInput(mFilename);
//what if an exception gets thrown in the line below?
reader = new BufferedReader(new InputStreamReader(in));
StringBuilder jsonString = new StringBuilder();
String line = null;
while ((line = reader.readLine()) != null) {
// Line breaks are omitted and irrelevant
jsonString.append(line);
}
// Parse the JSON using JSONTokener
JSONArray array = (JSONArray) new JSONTokener(jsonString.toString()).nextValue();
// Build the array of crimes from JSONObjects
for (int i = 0; i < array.length(); i++) {
crimes.add(new Crime(array.getJSONObject(i)));
}
} catch (FileNotFoundException e) {
// Ignore this one; it happens when starting fresh
} finally {
if (reader != null)
reader.close();
}
return crimes;
}
First, I wondered why we call .close() just on the BufferedReader object and not on the InputStream and InputStreamReader objects. I checked the official Oracle documentation and skimmed through a couple of questions in stackoverflow and according to what I've read BufferedReader.close() takes care of releasing the resources acquired by InputStreamReader and InputStream, so I don't have to call .close() on them.
Is this correct?
Secondly, I thought what would happen if an exception got thrown after creating the InputStream object and before creating the BufferedReader object. That is, either the InputStreamReader(InputStream in) constructor or BufferedReader(Reader in) constructor throws an exception. In that case we have acquired a file resource with InputStream but the BufferedReader object is still null, so in the finally block, the .close() method will not be invoked and the InputStream will not release the resources it has acquired.
Then I read the Oracle documentation, and neither of those two constructors throw an exception according to it. So it's not possible for an exception to occur between those two lines, right?
My final question is "Does this piece of code make sure it releases all resources it acquires?"
First, I wondered why we call .close() just on the BufferedReader object and not on the InputStream and InputStreamReader objects. I checked the official Oracle documentation and skimmed through a couple of questions in stackoverflow and according to what I've read BufferedReader.close() takes care of releasing the resources acquired by InputStreamReader and InputStream, so I don't have to call .close() on them.
Is this correct?
Yes, it is.
My final question is "Does this piece of code make sure it releases all resources it acquires?"
Most probably, yes. Even if some odd exceptions occur when creating the InputStreamReader or the BufferedReader, after exiting the block where they're defined, before GC collects them, they will be automatically closed as well. Of course, it's usually not a good idea to count on the GC to cleanup for you so if you want to be 100% sure, you might want to do it yourself in the finally block.
Another easier way is to use a FileReader which will simplify the code by only using two objects:
BufferedReader reader = new BufferedReader(new FileReader(fileName));
(I am assuming you want to read from a file).
I want to optimise my file reader function but am not sure if it is best practice to declare the nulls outside of the try loop. Also, is looping and appending chars to a Stringbuffer considered bad practice? I would like to use the exception handling here, but maybe it is better to use another structure? any advice most welcome, thanks.
public String readFile(){
File f = null;
FileReader fr = null;
StringBuffer content = null;
try{
f = new File("c:/test.txt");
fr = new FileReader(f);
int c;
while((c = fr.read()) != -1){
if(content == null){
content = new StringBuffer();
}
content.append((char)c);
}
fr.close();
}
catch (Exception e) {
throw new RuntimeException("An error occured reading your file");
}
return content.toString();
}
}
Advice:
Indent your code properly. The stuff in your question looks like a dog's breakfast.
You don't need to initialize f inside the try / catch block. The constructor can't throw an Exception the way you are using it.
In fact, you don't need to declare it at all. Just inline the new File(...).
In fact, you don't even need to do that. Use the FileReader(String) constructor.
There's no point initializing the StringBuffer inside the loop. The potential performance benefit is small and only applies in the edge-case where the file is empty or doesn't exist. In all other cases, this is an anti-optimization.
Don't catch Exception. Catch the exceptions that you expect to be thrown and allow all other exceptions to propagate. The unexpected exceptions are going to be due to bugs in your program, and need to be handled differently from others.
When you catch an exception, don't throw away the evidence. For an unexpected exception, either print / log the exception, its message and its stacktrace, or pass it as the 'cause' of the exception that you throw.
The FileReader should be closed in a finally clause. In your version of the code, the FileReader won't be closed if there is an exception after the object has been created and before the close() call. That will result in a leaked file descriptor and could cause problems later in your application.
Better yet, use the new Java 7 "try with resource" syntax which takes care of closing the "resource" automatically (see below).
You are reading from the file one character at a time. This is very inefficient. You need to either wrap the Reader in a BufferedReader, or read a large number of characters at a time using (for example) read(char[], int, int)
Use StringBuilder rather than StringBuffer ... unless you need a thread-safe string assembler.
Wrapping exceptions in RuntimeException is bad practice. It makes it difficult for the caller to handle specific exceptions ... if it needs to ... and even makes printing of a decent diagnostic more difficult. (And that assumes that you didn't throw away the original exception like your code does.)
Note: if you follow the advice of point 8 and not 9, you will find that the initialization of fr to null is necessary if you open the file in the try block.
Here's how I'd write this:
public String readFile() throws IOException {
// Using the Java 7 "try with resource syntax".
try (FileReader fr = new FileReader("c:/test.txt")) {
BufferedReader br = new BufferedReader(fr);
StringBuilder content = new StringBuilder();
int c;
while ((c = br.read()) != -1) {
content.append((char)c);
}
return content.toString();
}
}
A further optimization would be to use File.length() to find out what the file size (in bytes) is and use that as the initial size of the StringBuilder. However, if the files are typically small this is likely to make the application slower.
public String readFile() {
File f = new File("/Users/Guest/Documents/workspace/Project/src/test.txt");
FileReader fr = null;
BufferedReader br = null;
StringBuilder content = new StringBuilder();;
try {
fr = new FileReader(f);
br = new BufferedReader(fr);
//int c;
//while ((c = fr.read()) != -1) {
//content.append((char) c);
//}
String line = null;
while((line = br.readLine()) != null) {
content.append(line);
}
fr.close();
br.close();
} catch (Exception e) {
// do something
}
return content.toString();
}
Use buffered reader and youll get 70%+ improvement, use string builder instead of string buffer unless you need syncronization.
ran it on a 10MB file 50 times and averaged
no need to put anything that does not need exception handling inside try
no need for that if clause because it will be true only once and so you're wasting time - checking it for every character
there is no runtime exceptions to throw.
results:
fastest combination to slowest:
string builder and buffered reader line by line: 211 ms
string buffer and buffered reader line by line: 213 ms
string builder and buffered reader char by char: 348 ms
string buffer and buffered reader char by char: 372 ms
string builder and file reader char by char: 878
string buffer and file reader char by char: 935 ms
string: extremely slow
so use string builder + buffered reader and make it read line by line for best results.
I am creating a java console application and i need to use a notepad (txt) file to store PIN, AccountNo and Balance.
When the user enters the PIN (from BufferedReader) it should get validated from the notepad file and login to thge system. However, the code fragment which I used below only displays Enter PIN, and when i click on Enter, the program exits.
Could you please tell me what is the mistake which I have done here?
BufferedReader getIt = new BufferedReader(new InputStreamReader(System.in));
String userPIN = "";
try {
// Open the file that is the first command line parameter
FileInputStream fstream = new FileInputStream(
"D:\\Studies\\BCAS\\HND\\Semester 1\\Programming Concepts\\Assignment\\AccountInfo.txt");
// Get the object of DataInputStream
DataInputStream in = new DataInputStream(fstream);
BufferedReader br = new BufferedReader(new InputStreamReader(in));
String strLine;
//Read File Line By Line
System.out.println("Enter PIN");
userPIN = getIt.readLine();
while ((strLine = br.readLine()) != null) {
// Print the content on the console#
if (userPIN.equals(strLine)) {
System.out.println("You have login!");
}
}
//Close the input stream
in.close();
} catch (Exception e) {//Catch exception if any
System.err.println("Error: " + e.getMessage());
}
There are quite a few reasons why the program can simply terminate, or appear to be successful in program execution (and hence terminate successfully without doing any useful work):
The contents of the file are read, but if the end of the stream has been reached, then the code simply does not handle the case gracefully (or even consider handling it). Specifically, the line while ((strLine = br.readLine()) != null) can result in a scenario where the body of the loop is not executed.
The else condition is absent inside the body of the while loop. If none of the entries provided by the user input match the contents of the file, the code does not handle the condition, and the result is ambiguous. For all practical purposes, it appears that login is successful if the provided PIN is not present in the file.
Avoid unnecessary decoration of Readers and Streams. BufferedReader in
= new BufferedReader(new FileReader("foo.in")); is sufficient to read character data from files, and it does provide the read and readline methods to use.
PS: It is recommended that you learn to program defensively, especially when dealing with files and streams. That would help isolating cases in your code that would result in ambiguious/vague conditions.
File.ReadAllLines would be more convenient in your case.. Wouldn't be wrong to introduce a user class either if your going the good ol' oop way.
If you want to do this by yourself, StreamReader (which takes FileInfo) is a good way to go. Try to create it by using the using statement (:)) which makes cleans up after your done.