这里详细说下mysql jdbc防止sql注入的原理
1.代码层手动过滤
首先看下com.mysql.jdbc.PreparedStatement的setString 源码，可以看到setString对参数做了一定的处理，比如增加单引号，对换行符等一些字符做了转义处理，也就是说mysql jdbc 封装了一些代码层手动过滤，
经过setString 方法对输入字符串参数进行处理后，消除了一部分sql注入的威胁

public void setString(int parameterIndex, String x) throws SQLException {
        synchronized(this.checkClosed().getConnectionMutex()) {
            if (x == null) {
                this.setNull(parameterIndex, 1);
            } else {
                this.checkClosed();
                int stringLength = x.length();
                StringBuffer buf;
                if (this.connection.isNoBackslashEscapesSet()) {
                    boolean needsHexEscape = this.isEscapeNeededForString(x, stringLength);
                    Object parameterAsBytes;
                    byte[] parameterAsBytes;
                    if (!needsHexEscape) {
                        parameterAsBytes = null;
                        buf = new StringBuffer(x.length() + 2);
                        buf.append('\'');
                        buf.append(x);
                        buf.append('\'');
                        if (!this.isLoadDataQuery) {
                            parameterAsBytes = StringUtils.getBytes(buf.toString(), this.charConverter, this.charEncoding, this.connection.getServerCharacterEncoding(), this.connection.parserKnowsUnicode(), this.getExceptionInterceptor());
                        } else {
                            parameterAsBytes = StringUtils.getBytes(buf.toString());
                        this.setInternal(parameterIndex, parameterAsBytes);
                    } else {
                        parameterAsBytes = null;
                        if (!this.isLoadDataQuery) {
                            parameterAsBytes = StringUtils.getBytes(x, this.charConverter, this.charEncoding, this.connection.getServerCharacterEncoding(), this.connection.parserKnowsUnicode(), this.getExceptionInterceptor());
                        } else {
                            parameterAsBytes = StringUtils.getBytes(x);
                        this.setBytes(parameterIndex, parameterAsBytes);
                    return;
                String parameterAsString = x;
                boolean needsQuoted = true;
                if (this.isLoadDataQuery || this.isEscapeNeededForString(x, stringLength)) {
                    needsQuoted = false;
                    buf = new StringBuffer((int)((double)x.length() * 1.1D));
                    buf.append('\'');
                    for(int i = 0; i < stringLength; ++i) {
                        char c = x.charAt(i);
                        switch(c) {
                        case '\u0000':
                            buf.append('\\');
                            buf.append('0');
                            break;
                        case '\n':
                            buf.append('\\');
                            buf.append('n');
                            break;
                        case '\r':
                            buf.append('\\');
                            buf.append('r');
                            break;
                        case '\u001a':
                            buf.append('\\');
                            buf.append('Z');
                            break;
                        case '"':
                            if (this.usingAnsiMode) {
                                buf.append('\\');
                            buf.append('"');
                            break;
                        case '\'':
                            buf.append('\\');
                            buf.append('\'');
                            break;
                        case '\\':
                            buf.append('\\');
                            buf.append('\\');
                            break;
                        case '¥':
                        case '₩':
                            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) == 92) {
                                    buf.append('\\');
                        default:
                            buf.append(c);
                    buf.append('\'');
                    parameterAsString = buf.toString();
                buf = null;
                byte[] parameterAsBytes;
                if (!this.isLoadDataQuery) {
                    if (needsQuoted) {
                        parameterAsBytes = StringUtils.getBytesWrapped(parameterAsString, '\'', '\'', this.charConverter, this.charEncoding, this.connection.getServerCharacterEncoding(), this.connection.parserKnowsUnicode(), this.getExceptionInterceptor());
                    } else {
                        parameterAsBytes = StringUtils.getBytes(parameterAsString, this.charConverter, this.charEncoding, this.connection.getServerCharacterEncoding(), this.connection.parserKnowsUnicode(), this.getExceptionInterceptor());
                } else {
                    parameterAsBytes = StringUtils.getBytes(parameterAsString);
                this.setInternal(parameterIndex, parameterAsBytes);
                this.parameterTypes[parameterIndex - 1 + this.getParameterIndexOffset()] = 12;
2.预编译功能

预编译的功能是在服务端开启的，mysql预编译功能默认是关闭的，通过对jdbc url 追加以下参数来开启 useServerPrepStmts=true&cachePrepStmts=true