java中读取文件方法大全
下文笔者讲述java代码读取文件方法大全，如下所示
方法一:使用FileInputStream
package file.io.readfile;
import java.io.*;
public class FileInputStreamTest {
public static void main(String[] args) throws Exception {
// testInputStream();
// testInputStream2();
testInputStream3();
// testInputStreamReader();
}
public static void testInputStream() throws Exception {
String path = "E:\\test\\test1.txt";
long length = new File(path).length();
byte[] buffer = new byte[(int)length];
FileInputStream fileInputStream = new FileInputStream(path);
int unread = (int)length;
// 如果是大文件，需要多次读，这里简单地一次读取文件全部内容
// 多次读取可以采用的方式:
// 分配一个大内存，每次读取通过指定偏移以及读取数据大小来移动偏移
// 全部数据使用一个大内存，分配一个小内存来存储每次读取的数据，然后把每次读取的数据复制到大内存
try (BufferedInputStream bf = new BufferedInputStream(fileInputStream)) {
bf.read(buffer, 0, unread);
}
System.out.println(new String(buffer));
}
public static void testInputStream2() throws Exception {
String path = "E:\\test\\test1.txt";
long length = new File(path).length();
byte[] buffer = new byte[(int)length];
FileInputStream fileInputStream = new FileInputStream(path);
int unread = (int)length;
// 如果是大文件，需要多次读，这里简单地一次读取文件全部内容
// 多次读取可以采用的方式:
// 分配一个大内存，每次读取通过指定偏移以及读取数据大小来移动偏移
// 全部数据使用一个大内存，分配一个小内存来存储每次读取的数据，然后把每次读取的数据复制到大内存
int n;
int pos = 0;
try (BufferedInputStream bf = new BufferedInputStream(fileInputStream)) {
while((n = bf.read(buffer, pos, unread)) > 0) {
pos += n;
unread = unread - n;
}
}
System.out.println(new String(buffer));
}
public static void testInputStream3() throws Exception {
String path = "E:\\test\\test1.txt";
long length = new File(path).length();
byte[] buffer = new byte[(int)length];
FileInputStream fileInputStream = new FileInputStream(path);
int unread = (int)length;
// 如果是大文件，需要多次读，这里简单地一次读取文件全部内容
// 多次读取可以采用的方式:
// 分配一个大内存，每次读取通过指定偏移以及读取数据大小来移动偏移
// 全部数据使用一个大内存，分配一个小内存来存储每次读取的数据，然后把每次读取的数据复制到大内存
int n;
int pos = 0;
byte[] readBuf = new byte[1024];
try (BufferedInputStream bf = new BufferedInputStream(fileInputStream)) {
while((n = bf.read(readBuf)) > 0) {
int size = Math.min(n, unread);
System.arraycopy(readBuf, 0, buffer, pos, size);
pos += n;
unread = unread - n;
// 考虑文件增大，比刚开始大小大的情况。
if (unread <= 0) {
break;
}
}
}
System.out.println(new String(buffer));
}
/**
* 使用字符流
* @throws Exception
*/
public static void testInputStreamReader() throws Exception {
String path = "E:\\test\\test1.txt";
long length = new File(path).length();
byte[] buffer = new byte[(int)length];
FileInputStream fileInputStream = new FileInputStream(path);
int unread = (int)length;
// 按行读取，读到的每一行不包括末尾的换行符
try (BufferedReader bf = new BufferedReader(new InputStreamReader(fileInputStream))) {
String line;
while ((line = bf.readLine()) != null) {
System.out.println(line);
}
}
System.out.println(new String(buffer));
}
}
1.1 小结 1）FileInputStream可以直接读取字节流，也可以转换成字符流读取。 2）可以读取大文件。 3）需要使用者是否资源。 4）是传统经典使用方式。
使用FileReader
package file.io.readfile;
import java.io.BufferedReader;
import java.io.FileReader;
public class FileReaderTest {
public static void main(String[] args) throws Exception {
testReader();
}
public static void testReader() throws Exception {
String path = "E:\\test\\test1.txt";
// 使用FileReader不能指定文件编码
try (BufferedReader bf = new BufferedReader(new FileReader(path))) {
String line;
while ((line = bf.readLine()) != null) {
System.out.println(line);
}
}
}
}
2.1 小结 字符流方式读取文件，适合按行读取情况
3.使用RandomAccessFile
package file.io.readfile;
import java.io.RandomAccessFile;
public class RandomAccessFileTest {
public static void main(String[] args) throws Exception {
test1();
}
public static void test1() throws Exception {
String path = "E:\\test\\test1.txt";
try (RandomAccessFile randomAccessFile = new RandomAccessFile(path, "rw")) {
// 获取文件大小
// 另一个获取文件大小的方法是new File(path).length()
// 通过打开文件流来获取文件大小的方法更好，原因有：
// 避免两次打开文件；
// 避免并发问题导致获取文件大小错误
long length = randomAccessFile.length();
byte[] buffer = new byte[(int)length];
// 可以使用readLine分行读取
// 如果是大文件，需要多次读取
// 严格来说，需要判断是否读取完了整个文件
randomAccessFile.read(buffer);
System.out.println(new String(buffer));
}
}
}
3.1 小结 1）可以读取大文件 2）可以随机读取
4.使用Scanner
package file.io.readfile;
import java.io.FileReader;
import java.util.Scanner;
public class ScannerTest {
public static void main(String[] args) throws Exception {
// test1();
// test2();
// test3();
test4();
}
public static void test1() throws Exception {
String path = "E:\\test\\test1.txt";
try (Scanner scanner = new Scanner(new FileReader(path))) {
// 按空格分隔
while (scanner.hasNext()) {
String line = scanner.next();
System.out.println(line);
}
}
}
public static void test2() throws Exception {
String path = "E:\\test\\test1.txt";
try (Scanner scanner = new Scanner(new FileReader(path))) {
// 按空格分隔
while (scanner.hasNextLine()) {
String line = scanner.nextLine();
System.out.println(line);
}
}
}
public static void test3() throws Exception {
String path = "E:\\test\\test1.txt";
try (Scanner scanner = new Scanner(new FileReader(path))) {
// 指定分隔符
scanner.useDelimiter("\n");
while (scanner.hasNext()) {
String line = scanner.next();
System.out.println(line);
}
}
}
public static void test4() throws Exception {
String path = "E:\\test\\test1.txt";
try (Scanner scanner = new Scanner(new FileReader(path))) {
// 指定分隔符
scanner.useDelimiter(" ");
while (scanner.hasNext()) {
String word = scanner.next();
System.out.println(word);
}
}
}
}
4.1 适合按行、按分隔符读取字符文件
5.使用java.nio.file.Files
package file.io.readfile; import java.io.BufferedReader; import java.nio.charset.StandardCharsets; import java.nio.file.Files; import java.nio.file.Paths; import java.util.list; import java.util.stream.Collectors; import java.util.stream.Stream; public class FilesTest { public static void main(String[] args) throws Exception { // test1(); // test2(); // test3(); // test4(); // test5(); test6(); } public static void test1() throws Exception { String path = "E:\\test\\test1.txt"; // 可以指定编码 // Files.readAllLines(Paths.get(path), StandardCharsets.UTF_8); List<String> lines = Files.readAllLines(Paths.get(path)); for (String line : lines) { System.out.println(line); } } public static void test2() throws Exception { String path = "E:\\test\\test1.txt"; // 可以指定编码 // Files.newBufferedReader(Paths.get(path), StandardCharsets.UTF_8); try (BufferedReader bf = Files.newBufferedReader(Paths.get(path))) { String line; while ((line = bf.readLine()) != null) { System.out.println(line); } } } public static void test3() throws Exception { String path = "E:\\test\\test1.txt"; // 可以指定编码 // Files.lines(Paths.get(path), StandardCharsets.UTF_8); Stream<String> stringStream = Files.lines(Paths.get(path)); stringStream.forEach(line -> { System.out.println(line); }); } public static void test4() throws Exception { String path = "E:\\test\\test1.txt"; // 可以指定编码 // Files.lines(Paths.get(path), StandardCharsets.UTF_8); Stream<String> stringStream = Files.lines(Paths.get(path)); // 保证处理顺序 stringStream.forEachOrdered(line -> { System.out.println(line); }); } public static void test5() throws Exception { String path = "E:\\test\\test1.txt"; // 可以指定编码 // Files.lines(Paths.get(path), StandardCharsets.UTF_8); Stream<String> stringStream = Files.lines(Paths.get(path)); List<String> list = stringStream.collect(Collectors.toList()); for (String line : list) { System.out.println(line); } } /** * 一次性读取整个文件 * @throws Exception */ public static void test6() throws Exception { String path = "E:\\test\\test1.txt"; // 可以指定编码 // Files.readAllLines(Paths.get(path), StandardCharsets.UTF_8); byte[] buffer = Files.readAllBytes(Paths.get(path)); // jdk11提供方法 // Files.readString(Paths.get(path)); System.out.println(new String(buffer)); } }
5.1 小结 1.适合按行读取文件 2.一次性读取整个文件，得到字节数组
6.使用com.google.common.io.Files
package file.io.readfile;
import com.google.common.io.Files;
import java.io.BufferedReader;
import java.io.File;
import java.nio.charset.StandardCharsets;
import java.util.List;
public class FileTest2 {
public static void main(String[] args) throws Exception {
// test1();
test2();
}
public static void test1() throws Exception {
String path = "E:\\test\\test1.txt";
List<String> lines = Files.readLines(new File(path), StandardCharsets.UTF_8);
for (String line : lines) {
System.out.println(line);
}
}
public static void test2() throws Exception {
String path = "E:\\test\\test1.txt";
try (BufferedReader bf = Files.newReader(new File(path), StandardCharsets.UTF_8)) {
String line;
while ((line = bf.readLine()) != null) {
System.out.println(line);
}
}
}
}
6.1 适合按行读取文件
7.使用org.apache.commons.io.FileUtils
package file.io.readfile;
import org.apache.commons.io.FileUtils;
import java.io.File;
import java.nio.charset.StandardCharsets;
import java.util.List;
public class FileUtilsTest {
public static void main(String[] args) throws Exception {
// test1();
// test2();
test3();
}
public static void test1() throws Exception {
String path = "E:\\test\\test1.txt";
// 用编码字面值指定编码，例如
// List<String> lines = FileUtils.readLines(new File(path), “UTF-8");
// 或者使用默认编码
// List<String> lines = FileUtils.readLines(new File(path))
List<String> lines = FileUtils.readLines(new File(path), StandardCharsets.UTF_8);
for (String line : lines) {
System.out.println(line);
}
}
public static void test2() throws Exception {
String path = "E:\\test\\test1.txt";
// 用编码字面值指定编码，例如
// FileUtils.readFileToString(new File(path), “UTF-8");
// 或者使用默认编码
// FileUtils.readFileToString(new File(path))
String content = FileUtils.readFileToString(new File(path), StandardCharsets.UTF_8);
System.out.println(content);
}
/**
* 读取整个文件到字节数组
* @throws Exception
*/
public static void test3() throws Exception {
String path = "E:\\test\\test1.txt";
byte[] buffer = FileUtils.readFileToByteArray(new File(path));
System.out.println(new String(buffer));
}
}
7.1 1.适合按行读取文件 2.一次性读取整个文件，得到字节数组或字符串
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