ServletOutputStream类 简要说明
public void println() throws IOException; public void println(String s) throws IOException; public void println(boolean b) throws IOException; public void println(char c) throws IOException; public void println(int i) throws IOException; public void println(long l) throws IOException; public void println(float f) throws IOException; public void println(double d) throws IOException;

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输出变量到输出流中，并增加一个回车换行符