I'm using an ObjectInputStream to call readObject for reading in serialized Objects. I would like to avoid having this method block, so I'm looking to use something like Inputstream.available().
InputStream.available() will tell you there are bytes available and that read() will not block. Is there an equivalent method for seriailzation that will tell you if there are Objects available and readObject will not block?
No. Although you could use the ObjectInputStream in another thread and check to see whether that has an object available. Generally polling isn't a great idea, particularly with the poor guarantees of InputStream.available.
The Java serialization API was not designed to support an available() function. If you implement your own object reader/writer functions, you can read any amount of data off the stream you like, and there is no reporting method.
So readObject() does not know how much data it will read, so it does not know how many objects are available.
As the other post suggested, your best bet is to move the reading into a separate thread.
I have an idea that by adding another InputStream into the chain one can make availability information readable by the client:
HACK!
InputStream is = ... // where we actually read the data
BufferedInputStream bis = new BufferedInputStream(is);
ObjectInputStream ois = new ObjectInputStream(bis);
if( bis.available() > N ) {
Object o = ois.readObject();
}
The tricky point is value of N. It should be big enough to cover both serialization header and object data. If those are varying wildly, no luck.
The BufferedInputStream works for me, and why not just check if(bis.available() > 0) instead of a N value, this works perfectly for me.
I think ObjectInputStream.readObject blocks(= waits until) when no input is to be read. So if there is any input at all in the stream aka if(bis.available() > 0) ObjectInputStream.readObject will not block. Keep in mind that ObjectInputStream.readObject might throw a ClassNotFoundException, and that is't a problem at all to me.
Related
I want to write ONLY the values of the data members of an object into a file, so here I can can't use serialization since it writes a whole lot other information which i don't need. Here's is what I have implemented in two ways. One using byte buffer and other without using it.
Without using ByteBuffer:
1st method
public class DemoSecond {
byte characterData;
byte shortData;
byte[] integerData;
byte[] stringData;
public DemoSecond(byte characterData, byte shortData, byte[] integerData,
byte[] stringData) {
super();
this.characterData = characterData;
this.shortData = shortData;
this.integerData = integerData;
this.stringData = stringData;
}
public static void main(String[] args) {
DemoSecond dClass= new DemoSecond((byte)'c', (byte)0x7, new byte[]{3,4},
new byte[]{(byte)'p',(byte)'e',(byte)'n'});
File checking= new File("c:/objectByteArray.dat");
try {
if (!checking.exists()) {
checking.createNewFile();
}
// POINT A
FileOutputStream bo = new FileOutputStream(checking);
bo.write(dClass.characterData);
bo.write(dClass.shortData);
bo.write(dClass.integerData);
bo.write(dClass.stringData);
// POINT B
bo.close();
} catch (FileNotFoundException e) {
System.out.println("FNF");
e.printStackTrace();
} catch (IOException e) {
System.out.println("IOE");
e.printStackTrace();
}
}
}
Using byte buffer: One more thing is that the size of the data members will always remain fixed i.e. characterData= 1byte, shortData= 1byte, integerData= 2byte and stringData= 3byte. So the total size of this class is 7byte ALWAYS
2nd method
// POINT A
FileOutputStream bo = new FileOutputStream(checking);
ByteBuffer buff= ByteBuffer.allocate(7);
buff.put(dClass.characterData);
buff.put(dClass.shortData);
buff.put(dClass.integerData);
buff.put(dClass.stringData);
bo.write(buff.array());
// POINT B
I want know which one of the two methods is more optimized? And kindly give the reason also.
The above class DemoSecond is just a sample class.
My original classes will be of size 5 to 50 bytes. I don't think here size might be the issue.
But each of my classes is of fixed size like the DemoSecond
Also there are so many files of this type which I am going to write in the binary file.
PS
if I use serialization it also writes the word "characterData", "shortData", "integerData","stringData" also and other information which I don't want to write in the file. What I am corcern here is about THEIR VALUES ONLY. In case of this example its:'c', 7, 3,4'p','e','n'. I want to write only this 7bytes into the file, NOT the other informations which is USELESS to me.
As you are doing file I/O, you should bear in mind that the I/O operations are likely to be very much slower than any work done by the CPU in your output code. To a first approximation, the cost of I/O is an amount proportional to the amount of data you are writing, plus a fixed cost for each operating system call made to do the I/O.
So in your case you want to minimise the number of operating system calls to do the writing. This is done by buffering data in the application, so the application performs few put larger operating system calls.
Using a byte buffer, as you have done, is one way of doing this, so your ByteBuffer code will be more efficient than your FileOutputStream code.
But there are other considerations. Your example is not performing many writes. So it is likely to be very fast anyway. Any optimisation is likely to be a premature optimisation. Optimisations tend to make code more complicated and harder to understand. To understand your ByteBuffer code a reader needs to understand how a ByteBuffer works in addition to everything they need to understand for the FileOutputStream code. And if you ever change the file format, you are more likely to introduce a bug with the ByteBuffer code (for example, by having a too small a buffer).
Buffering of output is commonly done. So it should not surprise you that Java already provides code to help you. That code will have been written by experts, tested and debugged. Unless you have special requirements you should always use such code rather than writing your own. The code I am referring to is the BufferedOutputStream class.
To use it simply adapt your code that does not use the ByteBuffer, by changing the line of your code that opens the file to
OutputStream bo = new BufferedOutputStream(new FileOutputStream(checking));
The two methods differ only in the byte buffer allocated.
If you are concerning about unnecessary write action to file, there is already a BufferedOutputStream you can use, for which buffer is allocated internally, and if you are writing to same outputstream multiple times, it is definitely more efficient than allocating buffer every time manually.
It would be simplest to use a DataOutputStream around a BufferedOutputStream around the FileOutputStream.
NB You can't squeeze 'shortData' into a byte. Use the various primitives of DataOutputStream, and use the corresponding ones of DataInputStream when reading them back.
I've been trying to figure out why a method I've written to read objects from a file didn't work and realized that the available() method of ObjectInputStream gave 0 even though the file wasn't fully read.
The method did work after I've used the FileInputStream available() method instead to determine the EOF and it worked!
Why doesn't the method work for ObjectInputStram while it works for FileInputStream?
Here's the code:
public static void getArrFromFile() throws IOException, ClassNotFoundException {
Product p;
FileInputStream in= new FileInputStream(fName);
ObjectInputStream input= new ObjectInputStream(in);
while(in.available()>0){
p=(Product)input.readObject();
if (p.getPrice()>3000)
System.out.println(p);
}
input.close();
P.S-
I've read that I should use the EOF exception instead of available() for this, but I just wanna know why this doesn't work.
Thanks a lot!!!
Because, as the javadoc tells, available() returns an estimation of the number of bytes that can be read without blocking. The base InputStream implementation always returns 0, because this is a valid estimation. But whatever it returns, the fact that it returns 0 doesn't mean that there is nothing to read anymore. Only that the stream can't guarantee that at least one byte can be read without blocking.
Although this is not documented clearly I have realized from experience that it has to do with dynamic data. If your class only contains statically typed data, then available() is able to estimate the size. If there are dynamic data in your object, like lists etc, then it is not possible to make that estimation.
The available() method just tells how many bytes can be read without blocking. It's not very useful in regular code, but people see the name and erroneously think it does something else.
So in short: don't use available(), it's not the right method to use. Streams indicate ending differently, such as returning -1 or in ObjectInputStream's case, throwing an EOFException.
I found some trick! If you still want to use .available() to read all objects to the end, you can add an integer (ex: out.writeInt(0)) before adding each Object (ex: out.writeObject(obj)) when you write to the file and also read integer before reading each Object. So .available() can read byte left on file and won't be crash! Hope it helps you!
Use available function of InputStream instead of ObjectInputStream. Then if there is any data, get them as an object.
Something like:
if(inputStreamObject.available() > 0){
Object anyName = objectInputStreamObject.readObject();
}
You can get the inputStreamObject directly from the Socket.
I used it and the problem solved.
In short I need to do two things with one stream.
I need to pass a stream through a method to see if the bytes of that stream are of a particular type.
I need to create a new class using that stream once that check is completed.
I'm very new to streams and I know that they are "one way streets." So I think I have a bad design in my code or something if I find myself needing to reuse a stream.
Here is a snippit of the logic:
byte[] header = new byte[1024];
//reads entire array or until EOF whichever is first
bis.mark(header.length);
bis.read(header);
if(isFileType(header)) {
bis.reset();
_data.put(fileName, new MyClass(bis)); // Stream is now closed...
methodForFinalBytes(bis);
} else {
// Do other stuff;
}
It depends entirely on whether the InputStream implementation supports mark(). See http://docs.oracle.com/javase/6/docs/api/java/io/InputStream.html#markSupported(). Calling reset() on a stream that doesn't support mark() may throw an exception.
BufferedInputStream and ByteArrayInputStream support mark(), but others don't.
Generally, you can't reset an InputStream to get back to the start. There are, however the mark() / reset() methods, which make a stream remember the current position and you can rewind the stream to the marked position with reset().
Problem is, they are optional and may not be supported by the particular stream class in use. BufferedInputStream does support mark() / reset() (although within buffer limits). You can wrap your InputStream in a BufferedInputStream, immediately mark() and then run your detection code (but make sure it does not read ahead further than the buffer size, you can specify the buffer size in the BufferedInputStream constrcutor). Then call reset() and really read the stream.
EDIT: If you use ByteArrayInputStream anyway, that one supports mark/reset over its entire length (naturally).
I am new to Java. I want to learn to use GZIPstreams. I already have tried this:
ArrayList<SubImage>myObject = new ArrayList<SubImage>(); // SubImage is a Serializable class
ObjectOutputStream compressedOutput = new ObjectOutputStream(
new BufferedOutputStream(new GZIPOutputStream(new FileOutputStream(
new File("....")))));
compressedOutput.writeObject(myObject);
and
ObjectInputStream compressedInput = new ObjectInputStream(
new BufferedInputStream(new GZIPInputStream(new FileInputStream(
new File("....")))));
myObject=(ArrayList<SubImage>)compressedInput.readObject();
When the program writes myObject to a file without throwing any exception, but when it reaches the line
myObject=(ArrayList<SubImage>)compressedInput.readObject();
it throws this exception:
Exception in thread "main" java.io.EOFException: Unexpected end of ZLIB input stream
How can I solve this problem?
You have to flush and close your outputstream. Otherwhise, at least, the BufferedOutputStream will not write everything to the file (it does in big chucks to avoid penalizing performance).
If you call compressedOutput.flush() and compressedOutput.close() it will suffice.
You can try writing a simple string object and checking if the file is well written.
How? If you write a xxx.txt.gz file you can open it with your preferred zip app and look at the xxx.txt. If the app complains, then the content is not full written.
Extended answer to a comment: compressing even more the data
Changing serialization
You could change the standard serialization of SubImage object if it's an object of your own. Check java.io.Serializable javadoc to know how to do it. It's pretty straightforward.
Writing just what you need
Serialization has the drawback that needs to write "it's a SubImage" just before every instance you write. It's not necessary if you know what's going to be there beforehand. So you could try to serialize it more manually.
To write your list, instead of writing an object write directly the values that conform your list. You will need just a DataOutputStream (but ObjectOutputStream is a DOS so you can use it anyway).
dos.writeInt(yourList.size()); // tell how many items
for (SubImage si: yourList) {
// write every field, in order (this should be a method called writeSubImage :)
dos.writeInt(...);
dos.writeInt(...);
...
}
// to read the thing just:
int size = dis.readInt();
for (int i=0; i<size; i++) {
// read every field, in the same order (this should be a method called readSubImage :)
dis.readInt(...);
dis.readInt(...);
...
// create the subimage
// add it to the list you are recreating
}
This method is more manual but if:
you know what's going to be written
you will not need this kind of serialization for many types
it's pretty affordable and definitively more compressed than the Serializable counterpart.
Have in mind that there are alternative frameworks to serialize objects or create string messages (XStream for xml, Google Protocol Buffers for binary messages, and so on). That frameworks could work directly to binary or writing a string that could be then written.
If your app will need more on this, or just curious, maybe you should look at them.
Alternative serialization frameworks
Just looked in SO and found several questions (and answers) addressing this issue:
https://stackoverflow.com/search?q=alternative+serialization+frameworks+java
I've found that XStream is pretty easy and straightforward to use. And JSON is a format pretty readable and succint (and Javascript compatible which could be a plus :).
I should go for:
Object -> JSON -> OutputStreamWriter(UTF-8) -> GZippedOutputStream -> FileOutputStream
I suppose this is the input counterpart of this question which I asked some time back:
Can I use both PrintWriter and BufferedOutputStream on the same outputstream?
Q1) I need to read both String lines and byte [] from the same inputstream. So can I use the scanner wrapper to read lines first and then use the inputstream directly to read byte []? Will it cause any conflict?
Q2) If there are no more references to the scanner object and it gets garbage collected, will it automatically close the connection?
Q3) If the answer to the first question is yes and the answer to the second question is no, once I am done with the reading I only have to call inputstream.close() and not Scanner right? (Because by then I won't have a handle to the scanner object anymore)
For 1), you can always read bytes and convert them to String using the encoding of your choice. I'm pretty sure this is what all "readers" to under the hood.
For 2), no, Scanner class doesn't override the finalize method so I'm pretty sure it doesn't close the handle (and it really shouldn't). The section on finalizers in the Effective Java book has a detailed explanation on this topic.
For 3), closing the Scanner would automatically close the underlying stream. I'm pretty sure this is how almost all I/O classes handle the passed in file/resource handle.
Q1) Yes, the scanner buffers its input so when you come to switch to a different stream some of the bytes you want may have been consumed.
If you can use the Scanner to read bytes, that is a better option.
Q2) The connection will be closed when it is cleaned up.
Q3) You only need to close the input stream as Scanner is a pure Java object (and an input) For Buffered outputs you need to call flush() or close() to ensure unwritten data is sent.