虽然无法确定特定代码的行为方式，但我想最好的方法是像您一样对其进行分析。FileChannel 虽然被认为更快，但实际上对您的情况没有帮助。但这可能不是因为从文件中读取，但是您对读取的内容进行了实际处理。
在处理文件时我想指出的一篇文章是
https://www.redgreencode.com/why-is-java-io-slow/
还有对应的 Github 代码库
Java IO benchmark
我想指出这段代码使用了两个世界的组合
fos = new FileOutputStream(outputFile);
outFileChannel = fos.getChannel();
bufferedWriter = new BufferedWriter(Channels.newWriter(outFileChannel, "UTF-8"));
既然是在你的情况下阅读的，我会考虑
File inputFile = new File("C:\\input.txt");
FileInputStream fis = new FileInputStream(inputFile);
FileChannel inputChannel = fis.getChannel();
BufferedReader bufferedReader = new BufferedReader(Channels.newReader(inputChannel,"UTF-8"));
我还将调整块大小，并且使用 Spring 批处理总是反复试验才能找到最佳位置。
在完全不相关的说明中，您无法使用 BufferedReader 问题的原因是由于字符加倍，我假设这种情况更常见于 ebcdic 字符。我将简单地运行这样的循环来识别麻烦制造者和从源头上消除。
import java.io.UnsupportedEncodingException;
public class EbcdicConvertor {
public static void main(String[] args) throws UnsupportedEncodingException {
int index = 0;
for (int i = -127; i < 128; i++) {
byte[] b = new byte[1];
b[0] = (byte) i;
String cp037 = new String(b, "CP037");
if (cp037.getBytes().length == 2) {
index++;
System.out.println(i + "::" + cp037);
}
}
System.out.println(index);
}
}
上面的答案没有测试我的实际假设。这是一个测量时间的实际程序。结果在 200 MB 的文件中不言自明
import java.io.File;
import java.io.FileInputStream;
import java.io.FileReader;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.regex.Pattern;
public class ReadComplexDelimitedFile {
private static long total = 0;
private static final Pattern DELIMITER_PATTERN = Pattern.compile("\\^\\|\\^");
private void readFileUsingScanner() {
String s;
try (Scanner stdin = new Scanner(new File(this.getClass().getResource("input.txt").getPath()))) {
while (stdin.hasNextLine()) {
s = stdin.nextLine();
String[] fields = DELIMITER_PATTERN.split(s, 0);
total = total + fields.length;
}
} catch (Exception e) {
System.err.println("Error");
}
}
private void readFileUsingCustomBufferedReader() {
try (BufferedReader stdin = new BufferedReader(new FileReader(new File(this.getClass().getResource("input.txt").getPath())))) {
String s;
while ((s = stdin.readLine()) != null) {
String[] fields = DELIMITER_PATTERN.split(s, 0);
total += fields.length;
}
} catch (Exception e) {
System.err.println("Error");
}
}
private void readFileUsingBufferedReader() {
try (java.io.BufferedReader stdin = new java.io.BufferedReader(new FileReader(new File(this.getClass().getResource("input.txt").getPath())))) {
String s;
while ((s = stdin.readLine()) != null) {
String[] fields = DELIMITER_PATTERN.split(s, 0);
total += fields.length;
}
} catch (Exception e) {
System.err.println("Error");
}
}
private void readFileUsingBufferedReaderFileChannel() {
try (FileInputStream fis = new FileInputStream(this.getClass().getResource("input.txt").getPath())) {
try (FileChannel inputChannel = fis.getChannel()) {
try (BufferedReader stdin = new BufferedReader(Channels.newReader(inputChannel, "UTF-8"))) {
String s;
while ((s = stdin.readLine()) != null) {
String[] fields = DELIMITER_PATTERN.split(s, 0);
total = total + fields.length;
}
}
} catch (Exception e) {
System.err.println("Error");
}
} catch (Exception e) {
System.err.println("Error");
}
}
private void readFileUsingBufferedReaderByteFileChannel() {
try (FileInputStream fis = new FileInputStream(this.getClass().getResource("input.txt").getPath())) {
try (FileChannel inputChannel = fis.getChannel()) {
try (BufferedReader stdin = new BufferedReader(Channels.newReader(inputChannel, "UTF-8"))) {
int b;
StringBuilder sb = new StringBuilder();
while ((b = stdin.read()) != -1) {
if (b == 10) {
total = total + DELIMITER_PATTERN.split(sb, 0).length;
sb = new StringBuilder();
} else {
sb.append((char) b);
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
} catch (Exception e) {
System.err.println("Error");
}
}
private void readFileUsingFileChannelStream() {
try (RandomAccessFile fis = new RandomAccessFile(new File(this.getClass().getResource("input.txt").getPath()), "r")) {
try (FileChannel inputChannel = fis.getChannel()) {
ByteBuffer byteBuffer = ByteBuffer.allocate(8192);
ByteBuffer recordBuffer = ByteBuffer.allocate(250);
int recordLength = 0;
while ((inputChannel.read(byteBuffer)) != -1) {
byte b;
byteBuffer.flip();
while (byteBuffer.hasRemaining() && (b = byteBuffer.get()) != -1) {
if (b == 10) {
recordBuffer.flip();
total = total + splitIntoFields(recordBuffer, recordLength);
recordBuffer.clear();
recordLength = 0;
} else {
++recordLength;
recordBuffer.put(b);
}
}
byteBuffer.clear();
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
private int splitIntoFields(ByteBuffer recordBuffer, int recordLength) {
byte b;
String[] fields = new String[17];
int fieldCount = -1;
StringBuilder sb = new StringBuilder();
for (int i = 0; i < recordLength - 1; i++) {
b = recordBuffer.get(i);
if (b == 94 && recordBuffer.get(++i) == 124 && recordBuffer.get(++i) == 94) {
fields[++fieldCount] = sb.toString();
sb = new StringBuilder();
} else {
sb.append((char) b);
}
}
fields[++fieldCount] = sb.toString();
return fields.length;
}
public static void main(String args[]) {
//JVM wamrup
for (int i = 0; i < 100000; i++) {
total += i;
}
// We know scanner is slow-Still warming up
ReadComplexDelimitedFile readComplexDelimitedFile = new ReadComplexDelimitedFile();
List<Long> longList = new ArrayList<>(50);
for (int i = 0; i < 50; i++) {
total = 0;
long startTime = System.nanoTime();
readComplexDelimitedFile.readFileUsingScanner();
long stopTime = System.nanoTime();
long timeDifference = stopTime - startTime;
longList.add(timeDifference);
}
System.out.println("Time taken for readFileUsingScanner");
longList.forEach(System.out::println);
// Actual performance test starts here
longList = new ArrayList<>(10);
for (int i = 0; i < 10; i++) {
total = 0;
long startTime = System.nanoTime();
readComplexDelimitedFile.readFileUsingBufferedReaderFileChannel();
long stopTime = System.nanoTime();
long timeDifference = stopTime - startTime;
longList.add(timeDifference);
}
System.out.println("Time taken for readFileUsingBufferedReaderFileChannel");
longList.forEach(System.out::println);
longList.clear();
for (int i = 0; i < 10; i++) {
total = 0;
long startTime = System.nanoTime();
readComplexDelimitedFile.readFileUsingBufferedReader();
long stopTime = System.nanoTime();
long timeDifference = stopTime - startTime;
longList.add(timeDifference);
}
System.out.println("Time taken for readFileUsingBufferedReader");
longList.forEach(System.out::println);
longList.clear();
for (int i = 0; i < 10; i++) {
total = 0;
long startTime = System.nanoTime();
readComplexDelimitedFile.readFileUsingCustomBufferedReader();
long stopTime = System.nanoTime();
long timeDifference = stopTime - startTime;
longList.add(timeDifference);
}
System.out.println("Time taken for readFileUsingCustomBufferedReader");
longList.forEach(System.out::println);
longList.clear();
for (int i = 0; i < 10; i++) {
total = 0;
long startTime = System.nanoTime();
readComplexDelimitedFile.readFileUsingBufferedReaderByteFileChannel();
long stopTime = System.nanoTime();
long timeDifference = stopTime - startTime;
longList.add(timeDifference);
}
System.out.println("Time taken for readFileUsingBufferedReaderByteFileChannel");
longList.forEach(System.out::println);
longList.clear();
for (int i = 0; i < 10; i++) {
total = 0;
long startTime = System.nanoTime();
readComplexDelimitedFile.readFileUsingFileChannelStream();
long stopTime = System.nanoTime();
long timeDifference = stopTime - startTime;
longList.add(timeDifference);
}
System.out.println("Time taken for readFileUsingFileChannelStream");
longList.forEach(System.out::println);
}
}
BufferedReader 写得很早，因此我们可以重写与此示例相关的一些部分。例如，我们不关心 \r 和 skipLF 或 skipCR 或那些东西
我们将读取文件（无需同步）
通过扩展不需要StringBuffer，即使可以使用StringBuilder。立即看到性能改进。
危险的 hack，删除同步并用 StringBuilder 替换 StringBuffer 未经适当测试且不知道自己在做什么，请勿使用它
public String readLine() throws IOException {
StringBuilder s = null;
int startChar;
bufferLoop:
for (; ; ) {
if (nextChar >= nChars)
fill();
if (nextChar >= nChars) { /* EOF */
if (s != null && s.length() > 0)
return s.toString();
else
return null;
}
boolean eol = false;
char c = 0;
int i;
/* Skip a leftover '\n', if necessary */
charLoop:
for (i = nextChar; i < nChars; i++) {
c = cb[i];
if (c == '\n') {
eol = true;
break charLoop;
}
}
startChar = nextChar;
nextChar = i;
if (eol) {
String str;
if (s == null) {
str = new String(cb, startChar, i - startChar);
} else {
s.append(cb, startChar, i - startChar);
str = s.toString();
}
nextChar++;
return str;
}
if (s == null)
s = new StringBuilder(defaultExpectedLineLength);
s.append(cb, startChar, i - startChar);
}
}
Java 8 英特尔 i5 12 GB RAM Windows 10
结果：
readFileUsingBufferedReaderFileChannel 花费的时间::
- 2581635057 1849820885 1763992972 1770510738 1746444157 1733491399
1740530125 1723907177 1724280512 1732445638
readFileUsingBufferedReader 所用时间
- 1851027073 1775304769 1803507033 1789979554 1786974538 1802675458
1789672780 1798036307 1789847714 1785302003
readFileUsingCustomBufferedReader 所用时间
- 1745220476 1721039975 1715383650 1728548462 1724746005 1718177466
1738026017 1748077438 1724608192 1736294175
readFileUsingBufferedReaderByteFileChannel 所用时间
- 2872857919 2480237636 2917488143 2913491126 2880117231 2904614745
2911756298 2878777496 2892169722 2888091211
readFileUsingFileChannelStream 所用时间
- 3039447073 2896156498 2538389366 2906287280 2887612064 2929288046
2895626578 2955326255 2897535059 2884476915
进程以退出代码 0 结束