visualvm profiles jvm heap found int array takes huge memory

Question

I have a Spring boot program using OpenJdk (jdk1.8) running at server, consuming about 200 or 300 million data from kafka and write to csv files each day. Less than 2 hours after starts up, it using more than 6GB memory. So I dump heap using jmap histo. And find that int[] array using 2.6GB and byte[] array using 1.3GB. But I defined neither int[] nor byte[] in my project. I'm using spring kafka(org.springframework.kafka, version2.3.3) consume kafka message, opencsv(com.opencsv, version4.6) write csv.

Any one knows the reason?

Below is part of my code:

public <T> Boolean parseDataToFile(String filePath, List<T> data) throws IOException, CsvDataTypeMismatchException, CsvRequiredFieldEmptyException {
    if (data == null || data.size() <= 0) {
        return false;
    }
    File file = new File(filePath);
    //创建父目录
    boolean mkdirs = file.getParentFile().mkdirs();
    Writer writer = null;
    try {
        writer = new FileWriter(filePath, true);
        StatefulBeanToCsv beanToCsv = new StatefulBeanToCsvBuilder(writer).withThrowExceptions(false).withSeparator(',').build();
        beanToCsv.write(data);
        return true;
    } finally {
        if (writer != null) {
                writer.flush();
                writer.close();
        }
    }
}

Addtion: at Instance view, most(more than 90%) of them are none-used(retained size are 0), so it can be GCed? But why not? What are these int[] byte[] data?

Answer 1

Thanks to @AlBlue and @JurajMartinka. I analysed the byte[] array and int[] array and found part of the answer. The int[] array which have 166921 instance, using 2.6GB memory(57.3%). Most of them have no reference: Some of them are used by kafka: Meanwhile spring boot loader:

On the other hand, the byte[] array have 39400 instance, using 1.3GB memory(28.8%). Most of them are kafka data: Some are other referred dependencies:

The real memory using by live objects are not big. MAT(The Eclipse Memory Analyzer tool) shows that only 43.6M are occupied.

Yet, there are still many questions needs to find out. Such as when to GC, where is netty used, etc.