java.io.FileNotFoundException: d:.png (文件名、目录名或卷标语法不正确。)

错误信息如下：

java.io.FileNotFoundException: d:.png (文件名、目录名或卷标语法不正确。)
	at java.io.RandomAccessFile.open(Native Method)
	at java.io.RandomAccessFile.<init>(RandomAccessFile.java:241)
	at javax.imageio.stream.FileImageOutputStream.<init>(FileImageOutputStream.java:69)
	at com.sun.imageio.spi.FileImageOutputStreamSpi.createOutputStreamInstance(FileImageOutputStreamSpi.java:55)
	at javax.imageio.ImageIO.createImageOutputStream(ImageIO.java:419)
	at javax.imageio.ImageIO.write(ImageIO.java:1530)
	at cn.com.servlet.Dwg.doPost(Dwg.java:46)
	at cn.com.servlet.Dwg.doGet(Dwg.java:20)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
	at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:958)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:452)
	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1087)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:637)
	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:318)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:744)

错误代码如下：

// ---------- 背景透明代码结束 -----------------
		// 绘制路径直线图示意图
		gh.setColor(Color.CYAN);
		Recursive.dwf(gh);
		gh.dispose();
		response.reset();
		ServletOutputStream sos = response.getOutputStream();
		File f=new File("d:\f.png");
		if (sos != null) {
			ImageIO.write(bi, "png", f);
			sos.close();
			return;
		}

 错误原因：

File f=new File("d:\f.png");

应该是 File f=new File("d:/f.png");