How to Read a Large File Efficiently with Java – 如何用Java有效地读取一个大文件

最后修改: 2013年 12月 26日


1. Overview


This tutorial will show how to read all the lines from a large file in Java in an efficient manner.


This article is part of the “Java – Back to Basic” tutorial here on Baeldung.

本文是Java – Back to Basic“教程的一部分,在Baeldung这里。

2. Reading in Memory


The standard way of reading the lines of the file is in memory – both Guava and Apache Commons IO provide a quick way to do just that:

读取文件行数的标准方式是在内存中–Guava和Apache Commons IO都提供了一种快速的方式来实现。

Files.readLines(new File(path), Charsets.UTF_8);
FileUtils.readLines(new File(path));

The problem with this approach is that all the file lines are kept in memory – which will quickly lead to OutOfMemoryError if the File is large enough.


For example – reading a ~1Gb file:

例如 – 阅读一个~1Gb的文件

public void givenUsingGuava_whenIteratingAFile_thenWorks() throws IOException {
    String path = ...
    Files.readLines(new File(path), Charsets.UTF_8);

This starts off with a small amount of memory being consumed: (~0 Mb consumed)

这开始时消耗了少量的内存。(~0 Mb consumed)

[main] INFO - Total Memory: 128 Mb
[main] INFO - Free Memory: 116 Mb

However, after the full file has been processed, we have at the end: (~2 Gb consumed)

然而,在整个文件被处理之后,我们在最后有。(~2 Gb 消耗)

[main] INFO - Total Memory: 2666 Mb
[main] INFO - Free Memory: 490 Mb

Which means that about 2.1 Gb of memory are consumed by the process – the reason is simple – the lines of the file are all being stored in memory now.


It should be obvious by this point that keeping in memory the contents of the file will quickly exhaust the available memory – regardless of how much that actually is.


What’s more, we usually don’t need all of the lines in the file in memory at once – instead, we just need to be able to iterate through each one, do some processing and throw it away. So, this is exactly what we’re going to do – iterate through the lines without holding all of them in memory.


3. Streaming Through the File


Let’s now look at a solution – we’re going to use a java.util.Scanner to run through the contents of the file and retrieve lines serially, one by one:


FileInputStream inputStream = null;
Scanner sc = null;
try {
    inputStream = new FileInputStream(path);
    sc = new Scanner(inputStream, "UTF-8");
    while (sc.hasNextLine()) {
        String line = sc.nextLine();
        // System.out.println(line);
    // note that Scanner suppresses exceptions
    if (sc.ioException() != null) {
        throw sc.ioException();
} finally {
    if (inputStream != null) {
    if (sc != null) {

This solution will iterate through all the lines in the file – allowing for processing of each line – without keeping references to them – and in conclusion, without keeping them in memory: (~150 Mb consumed)

这个解决方案将遍历文件中的所有行–允许处理每一行–而不保留对它们的引用–最后,不在内存中保留它们(~150 Mb的消耗)

[main] INFO - Total Memory: 763 Mb
[main] INFO - Free Memory: 605 Mb

4. Streaming With Apache Commons IO

4.用Apache Commons IO进行流式传输

The same can be achieved using the Commons IO library as well, by using the custom LineIterator provided by the library:

使用Commons IO库也可以实现同样的效果,方法是使用库中提供的自定义LineIterator

LineIterator it = FileUtils.lineIterator(theFile, "UTF-8");
try {
    while (it.hasNext()) {
        String line = it.nextLine();
        // do something with line
} finally {

Since the entire file is not fully in memory – this will also result in pretty conservative memory consumption numbers: (~150 Mb consumed)

由于整个文件不完全在内存中 – 这也将导致相当保守的内存消耗数字(~150 Mb 消耗)

[main] INFO - Total Memory: 752 Mb
[main] INFO - Free Memory: 564 Mb

5. Conclusion


This quick article shows how to process lines in a large file without iteratively, without exhausting the available memory – which proves quite useful when working with these large files.


The implementation of all these examples and code snippets can be found in our GitHub project – this is a Maven-based project, so it should be easy to import and run as it is.