确定传输编码看起来是否可能是有效的ascii文本，从而可以作为7位代码进行传输。
/** 来 自 n o w j a v a . c o m**/ /* * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, * software distributed under the License is distributed on an * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY * KIND, either express or implied. See the License for the * specific language governing permissions and limitations * under the License. */ //package com.nowjava; import java.io.BufferedInputStream; import java.io.InputStream; import java.io.IOException; public class Main { /** * Determine if the transfer encoding looks like it might be * valid ascii text, and thus transferable as 7bit code. In * order for this to be true, all characters must be valid * 7-bit ASCII code AND all line breaks must be properly formed * (JUST '\r\n' sequences). 7-bit transfers also * typically have a line limit of 1000 bytes (998 + the CRLF), so any * stretch of Characters longer than that will also force Base64 encoding. * * @param content An input stream for the content we're examining. * * @exception IOException */ public static String getBinaryTransferEncoding(InputStream content) throws IOException { // for efficiency, we'll read in blocks. BufferedInputStream in = new BufferedInputStream(content, 4096); int previousChar = 0; int span = 0; // span of characters without a line break. while (true) { int ch = in.read(); // if we hit an EOF here, we've only found valid text so far, so we can transfer this as // 7-bit ascii. /* *来 自 nowjava - 时 代 Java */ if (ch == -1) { return "7bit"; } // we found a newline, this is only valid if the previous char was the '\r' if (ch == '\n') { // malformed linebreak? force this to base64 encoding. if (previousChar != '\r') { return "base64"; } // hit a line end, reset our line length counter span = 0; } else { span++; // the text has long lines, we can't transfer this as unencoded text. if (span > 998) { return "base64"; } // non-ascii character, we have to transfer this in binary. if (!isAscii(ch)) { return "base64"; } } previousChar = ch; } } /** * Test to see if this string contains only US-ASCII (i.e., 7-bit * ASCII) Characters. * * @param s The test string. * * @return true if this is a valid 7-bit ASCII encoding, false if it * contains any non-US ASCII characters. */ static public boolean isAscii(String s) { for (int i = 0; i < s.length(); i++) { if (!isAscii(s.charAt(i))) { return false; } } return true; }