对于包含数百万条记录的循环 - java.lang.OutOfMemoryError：Java堆空间

Question

I'm getting OutOfMemoryError: Java heap space for the following code:

StringBuilder obj= new StringBuilder("");
InputStream in = new FileInputStream(new File(file));
br = new BufferedReader(new InputStreamReader(in), 102400);
for (String line; (line= br.readLine()) != null;) {
    for (i = 0; i < 3; i++) {
        for (j = 1; j < 5; j++){ 
            obj.append(line.charAt(j)); 
        }
    }   
    hashMap.put(obj.toString(), someValue);
    obj.setLength(0);
}

but,

StringBuilder obj= new StringBuilder("");
InputStream in = new FileInputStream(new File(file));
br = new BufferedReader(new InputStreamReader(in), 102400);
for (String line; (line= br.readLine()) != null;) {
    obj.setLength(0);
    for (i = 0; i < 3; i++) {
        for (j = 1; j < 5; j++){ 
            obj.append(line.charAt(j)); 
        }
    }   
    hashMap.put(obj.toString(), someValue);
}

resolved the error? What is the issue in placing obj.setLength(0) at the beginning or at the end?

The Exception:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOfRange(Unknown Source)
at java.lang.String.<init>(Unknown Source)
at java.io.BufferedReader.readLine(Unknown Source)
at java.io.BufferedReader.readLine(Unknown Source)

Answer 1

Because threshhold of heapspace is already reached till you put objects in hashmap (last line in your code when its not giving error). Any operation (not only setting the length to zero but any other operation that deals with heap) will cause heapspace to become outOfMemory.

Answer 2

Increase heapspace for your java.

Add -XmX 4096

If you are really interested with your heap space usage you can run jvisualvm located in your JDK_HOME/bin directory.

Answer 3

Go to cmd (command line, yes I'm assuming you're on windows ;) ).

Then type the following precisely.

java -Xmx4096

Type in

java

to find out all the other things you can do. I hope this is more clear.