The MyHTTP Example

The best way of learning any language or notation is to read some examples of it. This example is adapted from http://www.w3.org/Protocols/HTTP-NG/asn1.html, it's a simplified form of the the GET request from FHTTP.

For our example, suppose that we need to employ the following ASN.1 module MyHTTP.asn:

MyHTTP DEFINITIONS
AUTOMATIC TAGS ::=
BEGIN

   GetRequest ::= SEQUENCE {
      header-only  BOOLEAN,
      lock         BOOLEAN,
      accept-types AcceptTypes,
      url          Url,
      ...,
      timestamp    GeneralizedTime
   }
   
   AcceptTypes ::= SET {
      standards BIT STRING { html(0), plain-text(1), gif(2), jpeg(3) } (SIZE(4)) OPTIONAL,
      others    SEQUENCE OF VisibleString (SIZE(4)) OPTIONAL
   }
   
   Url ::= VisibleString (FROM("a".."z"|"A".."Z"|"0".."9"|"./-_~%#"))
   
   myRequest GetRequest ::= {
	header-only  TRUE,
	lock         FALSE,
	accept-types {
		standards { html, plain-text }
	},
	url          "www.asnlab.org",
	timestamp    "20100202020202Z"
   }
   
END

1. Select File > New > Project...

2. Under ASN.1 category, select ASN.1 Project, then click Next.

3. For project name, enter 'MyHTTP' and click Finish.

4. ASN.1 files may now be added to the project by copying to the source folder, or creating ASN.1 files from scratch by

5. Select File > New > Other...

6. Under the ASN.1 category, select ASN.1 Module, then click Next.

7. For module name, enter 'MyHTTP' and click Finish.

8. Enter the above content in the opening editor.

9. Save the ASN.1 file, then the compiled Java class files will be automatically generated.

10. Create a Java project test, and add the ASN.1 Java Runtime to the build path.

11. Copy the generated Java class files into proper package ("MyHTTP" in this case) folder in the project.

12. Create a TestMyHttp.java file in the project with the following contents:

import java.io.ByteArrayOutputStream;
import java.util.Calendar;

import MyHTTP.AcceptTypes;
import MyHTTP.GetRequest;
import MyHTTP.Standards;


public class TestMyHttp {

	public static void main(String[] args) throws Exception {
		GetRequest getRequest=new GetRequest();
		getRequest.header_only=true;
		getRequest.lock=false;
		getRequest.accept_types=new AcceptTypes();
		getRequest.accept_types.standards=new Standards(new byte[1],(byte)4);
		getRequest.accept_types.standards.setHtml();
		getRequest.accept_types.standards.setPlain_text();
		getRequest.url="www.asnlab.org";
		Calendar cal = Calendar.getInstance();
		cal.set(Calendar.YEAR, 2010);
		cal.set(Calendar.MONTH, Calendar.FEBRUARY);
		cal.set(Calendar.DAY_OF_MONTH, 2);
		cal.set(Calendar.HOUR_OF_DAY, 2);
		cal.set(Calendar.MINUTE, 2);
		cal.set(Calendar.SECOND, 2);
		cal.set(Calendar.MILLISECOND, 0);
		cal.set(Calendar.ZONE_OFFSET, 0);
		getRequest.timestamp = cal.getTime();
		ByteArrayOutputStream bos=new ByteArrayOutputStream();
		getRequest.ber_encode(bos);
		byte[] bs=bos.toByteArray();
		for(int i=0; i<bs.length; i++) {
			System.out.printf("%02X ", bs[i] & 0xFF);
		}
	}

}

13. Compile the project if the Build automatically is turn off.

14. Right click the file TestMyHttp.java and select Run as > Java Application:

30 2D 80 01 FF 81 01 00 A2 04 80 02 04 C0 83 0E 77 77 77 2E 61 73 6E 6C 61 62 2E 63 6F 6D 84 0F 32 30 31 30 30 32 30 32 30 32 30 32 30 32 5A

Here's the explanation of encoded bits and bytes in BER:

0x30 -- [0011|0000], [UNIVERSAL, CONSTRUCTED, 16(SEQUENCE)] - GetRequest
0x2D -- [0010|1101], length 45

0x80 -- [1000|0000], [CONTEXT, PRIMITIVE, 0(BOOLEAN)] GetRequest.header_only
0x01 -- [0000|0001], length 1
0xFF -- [0000|1111], value TRUE

0x81 -- [1000|0001], [CONTEXT, PRIMITIVE, 1(BOOLEAN)] GetRequest.lock
0x01 -- [0000|0001], length 1
0x00 -- [0000|0000], value FALSE

0xA2 -- [1010|0010], [CONTEXT, CONSTRUCTED, 2(SET)] - GetRequest.accept_types
0x04 -- [0000|0100], length 4

0x80 -- [1000|0000], [CONTEXT, PRIMITIVE, 0(BIT STRING)] AcceptTypes.standards
0x02 -- [0000|0010], length 2
0x04 -- [0000|0100], 4 unused bits
0xC0 -- [1100|0000], {html, plaint_text}

0x83 -- [1000|0011], [CONTEXT, PRIMITIVE, 3(VisibleString)] GetRequest.url
0x0E -- [0000|1100], length 14
0x77 0x77 0x77 0x2E 0x61 0x73 0x6E 0x6C 0x61 0x62 0x2E 0x63 0x6F 0x6D -- www.asnlab.org

0x84 -- [1000|0011], [CONTEXT, PRIMITIVE, 4(GeneralizedTime)] GetRequest.timestamp
0x0F -- [0000|1100], length 15
0x32 0x30 0x31 0x30 0x30 0x32 0x30 0x32 0x30 0x32 0x30 0x32 0x30 0x32 0x5A -- 20100202020202Z