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jdbc中如何实现模糊查询
情况如何
再利用jdbc执行sql语句的时候，对于其他的句子的执行没什么太大的问题：加上占位符，然后设置占位符的值。
但是在模糊查询的时候，一直都写不对，这里提供了两种可选的解决办法，以供参考。
解决方法
第一种：
String sql = "select studentname, age, phone, address, other from customer"
+ " where studentname like ? ";
pstmt = conn.prepareStatement(sql);
// 设定参数
pstmt.setString(1, "%" + customername + "%" );
// 获取查询的结果集
rs = pstmt.executeQuery();
第二种：
百分号直接写在sql语句中
String sql = "select customercode, customername, phone, address, relationman, other from customer"
+ " where customername like \"%\"?\"%\" ";
pstmt = conn.prepareStatement(sql);
// 设定参数
pstmt.setString(1, customername);
// 获取查询的结果集
rs = pstmt.executeQuery();
为什么会这样？
得研究一下PreparedStatement是如何来处理占位符的。
在PresparedStatement中的setString()方法中有如下的一段代码：
public void setString(int parameterIndex, String x) throws SQLException {
synchronized (checkClosed().getConnectionMutex()) {
// 如果x是空的话,就直接调用另外的方法
if (x == null) {
setNull(parameterIndex, Types.CHAR);
} else {
checkClosed();
// 获得待插入的字符串的长度
int stringLength = x.length();
if (this.connection.isNoBackslashEscapesSet()) {
// Scan for any nasty chars
// 判断字符串中是否需要进行转义的字符
boolean needsHexEscape = isEscapeNeededForString(x, stringLength);
// 如果x里面没有需要转义的字符串
if (!needsHexEscape) {
byte[] parameterAsBytes = null;
StringBuilder quotedString = new StringBuilder(x.length() + 2);
// 直接就以字符串的形式加入到串中
quotedString.append(‘\‘‘);
quotedString.append(x);
quotedString.append(‘\‘‘);
if (!this.isLoadDataQuery) {
parameterAsBytes = StringUtils.getBytes(quotedString.toString(), this.charConverter, this.charEncoding,
this.connection.getServerCharset(), this.connection.parserKnowsUnicode(), getExceptionInterceptor());
} else {
// Send with platform character encoding
parameterAsBytes = StringUtils.getBytes(quotedString.toString());
}
setInternal(parameterIndex, parameterAsBytes);
} else {
byte[] parameterAsBytes = null;
if (!this.isLoadDataQuery) {
parameterAsBytes = StringUtils.getBytes(x, this.charConverter, this.charEncoding, this.connection.getServerCharset(),
this.connection.parserKnowsUnicode(), getExceptionInterceptor());
} else {
// Send with platform character encoding
parameterAsBytes = StringUtils.getBytes(x);
}
setBytes(parameterIndex, parameterAsBytes);
}
return;
}
String parameterAsString = x;
boolean needsQuoted = true;
if (this.isLoadDataQuery || isEscapeNeededForString(x, stringLength)) {
needsQuoted = false; // saves an allocation later
StringBuilder buf = new StringBuilder((int) (x.length() * 1.1));
buf.append(‘\‘‘);
//
// Note: buf.append(char) is _faster_ than appending in blocks, because the block append requires a System.arraycopy().... go figure...
//
// 如果需要转义则遍历需要转义的字符
for (int i = 0; i < stringLength; ++i) {
char c = x.charAt(i);
switch (c) {
case 0: /* Must be escaped for ‘mysql‘ */
buf.append(‘\\‘);
buf.append(‘0‘);
break;
case ‘\n‘: /* Must be escaped for logs */
buf.append(‘\\‘);
buf.append(‘n‘);
break;
case ‘\r‘:
buf.append(‘\\‘);
buf.append(‘r‘);
break;
case ‘\\‘:
buf.append(‘\\‘);
buf.append(‘\\‘);
break;
case ‘\‘‘:
buf.append(‘\\‘);
buf.append(‘\‘‘);
break;
case ‘"‘: /* Better safe than sorry */
if (this.usingAnsiMode) {
buf.append(‘\\‘);
}
buf.append(‘"‘);
break;
case ‘\032‘: /* This gives problems on Win32 */
buf.append(‘\\‘);
buf.append(‘Z‘);
break;
case ‘\u00a5‘:
case ‘\u20a9‘:
// escape characters interpreted as backslash by mysql
if (this.charsetEncoder != null) {
CharBuffer cbuf = CharBuffer.allocate(1);
ByteBuffer bbuf = ByteBuffer.allocate(1);
cbuf.put(c);
cbuf.position(0);
this.charsetEncoder.encode(cbuf, bbuf, true);
if (bbuf.get(0) == ‘\\‘) {
buf.append(‘\\‘);
}
}
// fall through
default:
buf.append(c);
}
}
buf.append(‘\‘‘);
parameterAsString = buf.toString();
}
byte[] parameterAsBytes = null;
if (!this.isLoadDataQuery) {
if (needsQuoted) {
parameterAsBytes = StringUtils.getBytesWrapped(parameterAsString, ‘\‘‘, ‘\‘‘, this.charConverter, this.charEncoding,
this.connection.getServerCharset(), this.connection.parserKnowsUnicode(), getExceptionInterceptor());
} else {
parameterAsBytes = StringUtils.getBytes(parameterAsString, this.charConverter, this.charEncoding, this.connection.getServerCharset(),
this.connection.parserKnowsUnicode(), getExceptionInterceptor());
}
} else {
// Send with platform character encoding
parameterAsBytes = StringUtils.getBytes(parameterAsString);
}
setInternal(parameterIndex, parameterAsBytes);
this.parameterTypes[parameterIndex - 1 + getParameterIndexOffset()] = Types.VARCHAR;
}
}
}
protected final void setInternal(int paramIndex, byte[] val) throws SQLException {
synchronized (checkClosed().getConnectionMutex()) {
int parameterIndexOffset = getParameterIndexOffset();
checkBounds(paramIndex, parameterIndexOffset);
this.isStream[paramIndex - 1 + parameterIndexOffset] = false;
this.isNull[paramIndex - 1 + parameterIndexOffset] = false;
this.parameterStreams[paramIndex - 1 + parameterIndexOffset] = null;
this.parameterValues[paramIndex - 1 + parameterIndexOffset] = val;
}
}
在setString()方法中，字符串会变为\‘string\‘的这种形式插入。
第一种方法可以成功比较好理解一些，但是第二种就有点想不通了。这里从源代码看出一点端倪就是会判断字符串中有没有转义字符，而且还会判断字符串需不需要被括起来。现就了解了这些，有空再深钻。
jdbc中如何实现模糊查询