I'm trying to understand what are the ramifications if the BufferedReader is never closed?
I just realized that many of the static code has a BufferedReader and they are never closed with try...catch..finally (br.close)
most of the code looks like this:
public static boolean isProcessX24Success(String data) throws Exception
{
BufferedReader br = new BufferedReader(new StringReader(data));
String line = "";
String result = "";
while ((line = br.readLine()) != null)
{
if (line.contains("SUCCESS X24"))
return true;
}
return false;
}
As you can see there's no br.close in a finally. Does that present a problem? if so what is it? the call to this method (and other from such nature) is done sequentially --not by multiple threads.
No problems because you use it on a StringReader. When the scope of the BufferedReder is left it will be eligible for garbage collection.
It would be a problem if you did not close e.g a FileReader because the system resource would not be closed properly and there might be a read-lock on the file until the program ends.
When writing it might be a problem, because the data might never be written to the file because of buffering.
Related
I want to listen for a named pipe on Linux (and only Linux) using Kotlin or Java. Currently I do the following:
val reader = File(PIPE_NAME).bufferedReader()
while (true) {
reader.readLine()?.let { string ->
// Next actions
}
This approach works, however the downside is that if nothing is available at the moment, readLine will return null, so the loop will consume the CPU.
Adding sleep is not an option as it's critical to get data ASAP.
So, the question is — is it possible to somehow read lines from a named pipe only when they're available and block otherwise?
If I understood properly, the problem starts when other side closes pipe, we may overcome that by opening pipe in rw mode:
try (RandomAccessFile raf = new RandomAccessFile(PIPE_NAME, "rw")) {
String str;
while ((str = raf.readLine()) != null) {
System.out.println(str);
}
}
in that case you will never get EOF since pipe is always open for write, thus the while loop never exits, however I do believe that is exactly what you want to achieve.
Another options is to reopen pipe after EOF, i.e.:
while (true) {
try (BufferedReader reader = new BufferedReader(new FileReader(PIPE_NAME))) {
String str;
while ((str = reader.readLine()) != null) {
System.out.println(str);
}
}
}
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 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.)
I am trying to read text from a web document using a BufferedReader over an InputStreamReader on an URL (to the file on some Apache server).
String result = "";
URL url = new URL("http://someserver.domain/somefile");
BufferedReader in = null;
in = new BufferedReader(new InputStreamReader(url.openStream(), "iso-8859-1"));
result += in.readLine();
Now this works just fine. But Obviously I'd like the reader not to just read one line, but as many as there are in the file.
Looking at the BufferedReader API the following code should do just that:
while (in.ready()) {
result += in.readLine();
}
I.e. read all lines while there are more lines, stop when no more lines are there. This code does not work however - the reader just never reports ready() = true!
I can even print the ready() value right before reading a line (which reads the correct string from the file) but the reader will report 'false'.
Am I doing something wrong? Why does the BufferedReader return 'false' on ready when there is actually stuff to read?
ready() != has more
ready() does not indicate that there is more data to be read. It only shows if a read will could block the thread. It is likely that it will return false before you read all data.
To find out if there is no more data check if readLine() returns null.
String line = in.readLine();
while(line != null){
...
line = in.readLine();
}
Another way you can do this that bypasses the in.ready() is something like:
while ((nextLine = in.readLine()) != null) {
result += nextLine;
}
You will just continue reading until you are done. This way you do not need to worry about the problem with in.ready().
I think the standard way to write this is to just attempt to read the line and verify that it returned sometime. Something like this:
while ((String nextLine = in.readLine()) != null) {
//System.out.println(nextLine);
result += nextLine;
}
So you just continue to go until you get null returned from the stream. See here for extra information:
http://download.oracle.com/javase/1.5.0/docs/api/java/io/BufferedReader.html#readLine()
The BufferedReader.ready() method is behaving as specified:
The Reader.ready() javadoc says the following:
[Returns] true if the next read() is guaranteed not to block for input, false otherwise. Note that returning false does not guarantee that the next read will block.
Then the BufferedReader.ready() javadoc says the following:
Tells whether this stream is ready to be read. A buffered character stream is ready if the buffer is not empty, or if the underlying character stream is ready.
If you put these two together, it is clear that BufferedReader.ready() can return false in situations where are characters available. In short, you shouldn't rely on ready() to test for logical end-of-file or end-of-stream.
This is what we have been using consistently for years - not sure if it is the "standard" method. I'd like to hear comments about the pros and cons of using URL.openURLStream() directly, and if that is causing the OP's problems. This code works for both HTTP and HTTPS connections.
URL getURL = new URL (servletURL.toString() + identifier+"?"+key+"="+value);
URLConnection uConn = getURL.openConnection();
BufferedReader br = new BufferedReader (new
InputStreamReader (uConn.getInputStream()));
for (String s = br.readLine() ; s != null ; s = br.readLine()) {
System.out.println ("[ServletOut] " + s);
// do stuff with s
}
br.close();
Basically the BufferedReader.ready() method can be used for checking whether the underlying stream is ready for providing data to the method caller.... else we can wait the thread for some time till it becomes ready.
But the real problem is that after we completely read the data stream, it will throw false..
so we didn't know whether the stream is fully read OR underlying stream is busy....
If you want to use in.ready(), the following worked for me well:
for (int i = 0; i < 10; i++) {
System.out.println("is InputStreamReader ready: " + in.ready());
if (!in.ready()) {
Thread.sleep(1000);
} else {
break;
}
}