From https://javaevangelist.blogspot.com/2006/11/how-to-convert-inputstream-to-string.html

IOUtils.java

package com.bluelotussoftware.io;

import java.io.ByteArrayInputStream;
import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.UnsupportedEncodingException;

/**
 *
 * @author John Yeary <jyeary@bluelotussoftware.com>
 * @version 2.0.0
 */
public class IOUtils {

    /**
     * This reads an {@link InputStream} and converts it to a {@link String}.
     *
     * @param is The stream to convert.
     * @return The converted {@link InputStream} to text.
     * @since 1.0.0
     */
    public String parseISToString(InputStream is) {
        DataInputStream din = new DataInputStream(is);
        StringBuilder sb = new StringBuilder();
        try {
            String line = null;
            while ((line = din.readLine()) != null) {
                sb.append(line).append("\n");
            }
        } catch (IOException ex) {
            ex.printStackTrace(System.err);
        } finally {
            try {
                is.close();
            } catch (IOException ex) {
            }
        }
        return sb.toString();
    }

    /**
     * Converts a {@link String} into an {@link InputStream}
     *
     * @param text The text to convert to an {@link InputStream}.
     * @return An {@link InputStream} from the text value provided.
     * @since 1.0.0
     */
    public InputStream parseStringToIS(String text) {
        if (text == null) {
            return null;
        }
        text = text.trim();
        java.io.InputStream in = null;
        try {
            in = new ByteArrayInputStream(text.getBytes("UTF-8"));
        } catch (UnsupportedEncodingException ex) {
            ex.printStackTrace(System.err);
        }
        return in;
    }

    /**
     * This reads an {@link InputStream} and converts it to a {@link String}.
     *
     * @param is The stream to convert.
     * @return The converted {@link InputStream} to UTF-8 text.
     * @since 2.0.0
     */
    public String toString(final InputStream is) {
        StringBuilder sb = new StringBuilder();
        try (DataInputStream din = new DataInputStream(is)) {
            String line;
            while ((line = din.readUTF()) != null) {
                sb.append(line);
            }
        } catch (IOException ex) {
            ex.printStackTrace(System.err);
        }
        return sb.toString();
    }

    /**
     * Converts a {@link String} into an {@link InputStream}
     *
     * @param text The text to convert to an {@link InputStream}.
     * @return A UTF-8 encoded {@link InputStream} from the text value provided.
     * @since 2.0.0
     */
    public InputStream toInputStream(final String text) {
        if (text == null || text.isEmpty()) {
            return null;
        }

        try (InputStream in = new ByteArrayInputStream(text.getBytes("UTF-8"))) {
            return in;
        } catch (UnsupportedEncodingException ex) {
            ex.printStackTrace(System.err);
        } catch (IOException ex) {
            ex.printStackTrace(System.err);
        }
        return null;
    }
}