Spring 中StreamUtils教程

本文我们介绍StreamUtils类使用。StreamUtils是spring中用于处理流的类，是java.io包中inputStream和outputStream，不是java8中Steam。使用时仅依赖spring-core，主要需要了解几个静态方法。

拷贝Stream

StreamUtils类包括几个重载copy()方法，也包括几个变体：

• copyRange()
• copyToByteArray()
• copyString()

我们能不依赖任何jar实现拷贝流。然而，这样会使代码冗难读，不易理解。

为了简单起见，我们省略关闭流，实际应用中需要调用close()方法关闭输入或输出流。

让我们看看如何拷贝inputStream内容至outputStream：

@Test
public void whenCopyInputStreamToOutputStream_thenCorrect() throws IOException {
    String inputFileName = "src/test/resources/input.txt";
    String outputFileName = "src/test/resources/output.txt";
    File outputFile = new File(outputFileName);
    InputStream in = new FileInputStream(inputFileName);
    OutputStream out = new FileOutputStream(outputFile);

    StreamUtils.copy(in, out);

    assertTrue(outputFile.exists());
    String inputFileContent = getStringFromInputStream(new FileInputStream(inputFileName));
    String outputFileContent = getStringFromInputStream(new FileInputStream(outputFileName));
    assertEquals(inputFileContent, outputFileContent);
}

创建的新文件包括inputStream中的内容。

其中getStringFromInputStream() 方法负责取inputStream内容并返回字符串。

import org.apache.commons.io.IOUtils;
import org.springframework.util.StreamUtils;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.StringWriter;

public class CopyStream {
    public static String getStringFromInputStream(InputStream input) throws IOException {
        StringWriter writer = new StringWriter();
        IOUtils.copy(input, writer, "UTF-8");
        return writer.toString();
    }

    public InputStream getNonClosingInputStream() throws IOException {
        InputStream in = new FileInputStream("src/test/resources/input.txt");
        return StreamUtils.nonClosing(in);
    }
}

我们也可以不拷贝inputStream全部内容，可以使用copyRange()方法仅拷贝一定范围的内容至outputStream.

@Test
public void whenCopyRangeOfInputStreamToOutputStream_thenCorrect() throws IOException {
    String inputFileName = "src/test/resources/input.txt";
    String outputFileName = "src/test/resources/output.txt";
    File outputFile = new File(outputFileName);
    InputStream in = new FileInputStream(inputFileName);
    OutputStream out = new FileOutputStream(outputFileName);

    StreamUtils.copyRange(in, out, 1, 10);

    assertTrue(outputFile.exists());
    String inputFileContent = getStringFromInputStream(new FileInputStream(inputFileName));
    String outputFileContent = getStringFromInputStream(new FileInputStream(outputFileName));

    assertEquals(inputFileContent.substring(1, 11), outputFileContent);
}

如上述代码所示，copyRange方法有四个参数，inputStream，outputStream，开始拷贝位置，结束拷贝位置。如果我们指定的长度超过inputStream的长度呢？copyRange方法仅拷贝至流的结尾。

下面看看如何拷贝字符串至outputStream：

@Test
public void whenCopyStringToOutputStream_thenCorrect() throws IOException {
    String string = "Should be copied to OutputStream.";
    String outputFileName = "src/test/resources/output.txt";
    File outputFile = new File(outputFileName);
    OutputStream out = new FileOutputStream("src/test/resources/output.txt");

    StreamUtils.copy(string, StandardCharsets.UTF_8, out);

    assertTrue(outputFile.exists());

    String outputFileContent = getStringFromInputStream(new FileInputStream(outputFileName));

    assertEquals(outputFileContent, string);
}

copy方法带三个参数：被拷贝的字符串，写文件时指定的字符集，指定目的地（outputStream）。

我们也可以把给定的inputStream内容拷贝为字符串：

@Test
public void whenCopyInputStreamToString_thenCorrect() throws IOException {
    String inputFileName = "src/test/resources/input.txt";
    InputStream is = new FileInputStream(inputFileName);
    String content = StreamUtils.copyToString(is, StandardCharsets.UTF_8);

    String inputFileContent = getStringFromInputStream(new FileInputStream(inputFileName));
    assertEquals(inputFileContent, content);
}

也可以拷贝字节数组的内容至outputStream：

public void whenCopyByteArrayToOutputStream_thenCorrect() throws IOException {
    String outputFileName = "src/test/resources/output.txt";
    String string = "Should be copied to OutputStream.";
    byte[] byteArray = string.getBytes();
    OutputStream out = new FileOutputStream("src/test/resources/output.txt");

    StreamUtils.copy(byteArray, out);

    String outputFileContent = getStringFromInputStream(new FileInputStream(outputFileName));

    assertEquals(outputFileContent, string);
}

同样，也可以拷贝inputStream内容至字节数组：

public void whenCopyInputStreamToByteArray_thenCorrect() throws IOException {
    String inputFileName = "src/test/resources/input.txt";
    InputStream is = new FileInputStream(inputFileName);
    byte[] out = StreamUtils.copyToByteArray(is);

    String content = new String(out);
    String inputFileContent = getStringFromInputStream(new FileInputStream(inputFileName));

    assertEquals(inputFileContent, content);
}

其他功能

inputStream可以作为参数传递给drain()方法，删除流中所有剩余数据：

StreamUtils.drain(in);

也可以使用emptyInput()方法获得一个有效空输入流：

public InputStream getInputStream() {
    return StreamUtils.emptyInput();
}

还有两个重载方法nonClosing(),inputStream和outputStream流可以作为参数，用于返回无需关闭的inputStream和outputStream流。

public InputStream getNonClosingInputStream() throws IOException {
    InputStream in = new FileInputStream("src/test/resources/input.txt");
    return StreamUtils.nonClosing(in);
}

总结

本文介绍了StreamUtil类及其所有方法，利用可以大大简单代码，提高开发效率。