在JDK8

Stream<String> lines = Files.lines(path);
String data = lines.collect(Collectors.joining("\n"));
lines.close();

import java.io.File;
import java.nio.file.Files;
import java.nio.file.Path;

File file = getYourFile();
Path path = file.toPath();
byte[] data = Files.readAllBytes(path);

让我添加另一个不使用第三方库的解决方案。它重用了Scott提出的异常处理模式(链接)。我把丑陋的部分移动到一个单独的消息(我会隐藏在一些FileUtils类;))

public void someMethod() {
    final byte[] buffer = read(new File("test.txt"));
}

private byte[] read(final File file) {
    if (file.isDirectory())
        throw new RuntimeException("Unsupported operation, file "
                + file.getAbsolutePath() + " is a directory");
    if (file.length() > Integer.MAX_VALUE)
        throw new RuntimeException("Unsupported operation, file "
                + file.getAbsolutePath() + " is too big");

    Throwable pending = null;
    FileInputStream in = null;
    final byte buffer[] = new byte[(int) file.length()];
    try {
        in = new FileInputStream(file);
        in.read(buffer);
    } catch (Exception e) {
        pending = new RuntimeException("Exception occured on reading file "
                + file.getAbsolutePath(), e);
    } finally {
        if (in != null) {
            try {
                in.close();
            } catch (Exception e) {
                if (pending == null) {
                    pending = new RuntimeException(
                        "Exception occured on closing file" 
                             + file.getAbsolutePath(), e);
                }
            }
        }
        if (pending != null) {
            throw new RuntimeException(pending);
        }
    }
    return buffer;
}

基本上你必须在记忆中读它。打开文件，分配数组，并将文件中的内容读入数组。

最简单的方法是这样的:

public byte[] read(File file) throws IOException, FileTooBigException {
    if (file.length() > MAX_FILE_SIZE) {
        throw new FileTooBigException(file);
    }
    ByteArrayOutputStream ous = null;
    InputStream ios = null;
    try {
        byte[] buffer = new byte[4096];
        ous = new ByteArrayOutputStream();
        ios = new FileInputStream(file);
        int read = 0;
        while ((read = ios.read(buffer)) != -1) {
            ous.write(buffer, 0, read);
        }
    }finally {
        try {
            if (ous != null)
                ous.close();
        } catch (IOException e) {
        }

        try {
            if (ios != null)
                ios.close();
        } catch (IOException e) {
        }
    }
    return ous.toByteArray();
}

这对文件内容有一些不必要的复制(实际上数据复制了三次:从文件到缓冲区，从缓冲区到ByteArrayOutputStream，从ByteArrayOutputStream到实际的结果数组)。

你还需要确保你只在内存中读取一定大小的文件(这通常取决于应用程序):-)。

您还需要在函数外部处理IOException。

另一种方式是:

public byte[] read(File file) throws IOException, FileTooBigException {
    if (file.length() > MAX_FILE_SIZE) {
        throw new FileTooBigException(file);
    }

    byte[] buffer = new byte[(int) file.length()];
    InputStream ios = null;
    try {
        ios = new FileInputStream(file);
        if (ios.read(buffer) == -1) {
            throw new IOException(
                    "EOF reached while trying to read the whole file");
        }
    } finally {
        try {
            if (ios != null)
                ios.close();
        } catch (IOException e) {
        }
    }
    return buffer;
}

这没有不必要的复制。

FileTooBigException是自定义应用程序异常。 MAX_FILE_SIZE常量是一个应用程序参数。

对于大文件，您可能应该考虑流处理算法或使用内存映射(参见java.nio)。

// Returns the contents of the file in a byte array.
    public static byte[] getBytesFromFile(File file) throws IOException {        
        // Get the size of the file
        long length = file.length();

        // You cannot create an array using a long type.
        // It needs to be an int type.
        // Before converting to an int type, check
        // to ensure that file is not larger than Integer.MAX_VALUE.
        if (length > Integer.MAX_VALUE) {
            // File is too large
            throw new IOException("File is too large!");
        }

        // Create the byte array to hold the data
        byte[] bytes = new byte[(int)length];

        // Read in the bytes
        int offset = 0;
        int numRead = 0;

        InputStream is = new FileInputStream(file);
        try {
            while (offset < bytes.length
                   && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0) {
                offset += numRead;
            }
        } finally {
            is.close();
        }

        // Ensure all the bytes have been read in
        if (offset < bytes.length) {
            throw new IOException("Could not completely read file "+file.getName());
        }
        return bytes;
    }

如果你的目标版本少于26个API，试试这个

 private static byte[] readFileToBytes(String filePath) {

    File file = new File(filePath);
    byte[] bytes = new byte[(int) file.length()];

    // funny, if can use Java 7, please uses Files.readAllBytes(path)
    try(FileInputStream fis = new FileInputStream(file)){
        fis.read(bytes);
        return bytes;
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;

}