Snmp4j程式設計簡介之三：Snmp
Class Snmp
java.lang.Object
org.snmp4j.Snmp
All Implemented Interfaces:
java.util.EventListener, CommandResponder, Session
snmp類是SNMP4J的核心，它提供了發送和接收SNMP PDUs的方法，所有的SNMP PDU 類型都可以采用同步或者異步的方式被發送。
Snmp采用獨立的傳輸協定，通過TransportMapping 接口調用addTransportMapping(TransportMapping transportMapping) 方法或者采用預設的構造函數來實作傳輸映射，以此來實作資訊的傳輸。
下面的代碼片段是采用UDP傳輸方式來實作一個SNMPv3的SNMP session ：
Java代碼
Address targetAddress = GenericAddress.parse("udp:127.0.0.1/161");
TransportMapping transport = new DefaultUdpTransportMapping();
snmp = new Snmp(transport);
USM usm = new USM(SecurityProtocols.getInstance(),
new OctetString(MPv3.createLocalEngineID()), 0);
SecurityModels.getInstance().addSecurityModel(usm);
transport.listen();
Address targetAddress = GenericAddress.parse("udp:127.0.0.1/161");
TransportMapping transport = new DefaultUdpTransportMapping();
snmp = new Snmp(transport);
USM usm = new USM(SecurityProtocols.getInstance(),
new OctetString(MPv3.createLocalEngineID()), 0);
SecurityModels.getInstance().addSecurityModel(usm);
transport.listen();
如何實作SNMPv3資訊的同步發送，下面舉例說明：
Java代碼
// add user to the USM
snmp.getUSM().addUser(new OctetString("MD5DES"),
new UsmUser(new OctetString("MD5DES"),
AuthMD5.ID,
new OctetString("MD5DESUserAuthPassword"),
PrivDES.ID,
new OctetString("MD5DESUserPrivPassword")));
// create the target
UserTarget target = new UserTarget();
target.setAddress(targetAddress);
target.setRetries(1);
target.setTimeout(5000);
target.setVersion(SnmpConstants.version3);
target.setSecurityLevel(SecurityLevel.AUTH_PRIV);
target.setSecurityName(new OctetString("MD5DES"));
// create the PDU
PDU pdu = new ScopedPDU();
pdu.add(new VariableBinding(new OID("1.3.6")));
pdu.setType(PDU.GETNEXT);
// send the PDU
ResponseEvent response = snmp.send(pdu, target);
// extract the response PDU (could be null if timed out)
PDU responsePDU = response.getResponse();
// extract the address used by the agent to send the response:
Address peerAddress = response.getPeerAddress();
An asynchronous SNMPv1 request is sent by the following code:
// setting up target
CommunityTarget target = new CommunityTarget();
target.setCommunity(new OctetString("public"));
target.setAddress(targetAddress);
target.setRetries(2);
target.setTimeout(1500);
target.setVersion(SnmpConstants.version1);
// creating PDU
PDU pdu = new PDU();
pdu.add(new VariableBinding(new OID(new int[] {1,3,6,1,2,1,1,1})));
pdu.add(new VariableBinding(new OID(new int[] {1,3,6,1,2,1,1,2})));
pdu.setType(PDU.GETNEXT);
// sending request
ResponseListener listener = new ResponseListener() {
public void onResponse(ResponseEvent event) {
// Always cancel async request when response has been received
// otherwise a memory leak is created! Not canceling a request
// immediately can be useful when sending a request to a broadcast
// address.
((Snmp)event.getSource()).cancel(event.getRequest(), this);
System.out.println("Received response PDU is: "+event.getResponse());
}
};
snmp.sendPDU(pdu, target, null, listener);
//Traps (notifications) and other SNMP PDUs can be received by adding the folling code to the first code snippet above:
CommandResponder trapPrinter = new CommandResponder() {
public synchronized void processPdu(CommandResponderEvent e) {
PDU command = e.getPDU();
if (command != null) {
System.out.println(command.toString());
}
}
};
snmp.addCommandResponder(trapPrinter);
Version:
1.8
Author:
Frank Fock
translate：avery_leo
// add user to the USM
snmp.getUSM().addUser(new OctetString("MD5DES"),
new UsmUser(new OctetString("MD5DES"),
AuthMD5.ID,
new OctetString("MD5DESUserAuthPassword"),
PrivDES.ID,
new OctetString("MD5DESUserPrivPassword")));
// create the target
UserTarget target = new UserTarget();
target.setAddress(targetAddress);
target.setRetries(1);
target.setTimeout(5000);
target.setVersion(SnmpConstants.version3);
target.setSecurityLevel(SecurityLevel.AUTH_PRIV);
target.setSecurityName(new OctetString("MD5DES"));
// create the PDU
PDU pdu = new ScopedPDU();
pdu.add(new VariableBinding(new OID("1.3.6")));
pdu.setType(PDU.GETNEXT);
// send the PDU
ResponseEvent response = snmp.send(pdu, target);
// extract the response PDU (could be null if timed out)
PDU responsePDU = response.getResponse();
// extract the address used by the agent to send the response:
Address peerAddress = response.getPeerAddress();
An asynchronous SNMPv1 request is sent by the following code:
// setting up target
CommunityTarget target = new CommunityTarget();
target.setCommunity(new OctetString("public"));
target.setAddress(targetAddress);
target.setRetries(2);
target.setTimeout(1500);
target.setVersion(SnmpConstants.version1);
// creating PDU
PDU pdu = new PDU();
pdu.add(new VariableBinding(new OID(new int[] {1,3,6,1,2,1,1,1})));
pdu.add(new VariableBinding(new OID(new int[] {1,3,6,1,2,1,1,2})));
pdu.setType(PDU.GETNEXT);
// sending request
ResponseListener listener = new ResponseListener() {
public void onResponse(ResponseEvent event) {
// Always cancel async request when response has been received
// otherwise a memory leak is created! Not canceling a request
// immediately can be useful when sending a request to a broadcast
// address.
((Snmp)event.getSource()).cancel(event.getRequest(), this);
System.out.println("Received response PDU is: "+event.getResponse());
}
};
snmp.sendPDU(pdu, target, null, listener);
//Traps (notifications) and other SNMP PDUs can be received by adding the folling code to the first code snippet above:
CommandResponder trapPrinter = new CommandResponder() {
public synchronized void processPdu(CommandResponderEvent e) {
PDU command = e.getPDU();
if (command != null) {
System.out.println(command.toString());
}
}
};
snmp.addCommandResponder(trapPrinter);
Version:
1.8
Author:
Frank Fock
translate：avery_leo
Snmp類提供了一套有關Snmp的功能接口。具體來講，就是發送、接受、建立Snmp消息。
一個Snmp對象是一個Session，而在Snmp4j中，一個Session可以同多個遠端裝置通信。
（1） Snmp、Target、PDU三者的關系
Target代表遠端裝置或者遠端實體、PDU代表管理端同Target通信的資料，Snmp就代表管理者管理功能（其實就是資料的收發）的具體執行者。
打個比方：Target就是你遠方的戀人，PDU就是你們之間傳遞的情書、而Snmp就是負責幫你寄信收信的郵差。
（2）Snmp收發資料的兩種方式
Snmp可以同步、也可異步收發資料。詳細見代碼示例說明。
（3）Snmp與傳輸層協定
Snmp可以定制傳輸層協定，一般選擇udp，也可以選擇tcp。詳細見代碼示例說明。
（4）Snmp與Usm
建立Snmp用來發送Snmpv3版本的消息時候，一般還要建立USM，将它添加至安全模型管理器（SecriryModels）中，同時還需要向Usm 中添加相應的USM使用者（UsmUser）。詳細見代碼示例說明。
代碼示例：（摘自Snmp4j的API文檔）
（1）建立Snmp
1）使用UDP傳輸協定
TransportMapping transport = new DefaultUdpTransportMapping();
snmp = new Snmp(transport); 2）使用TCP傳輸協定
TransportMapping transport = new DefaultTcpTransportMapping();
snmp = new Snmp(transport); 3）建立用于Snmpv3的Snmp
// 建立Snmp
TransportMapping transport =
new DefaultUdpTransportMapping();
Snmp snmp = new Snmp(transport);
if (version == SnmpConstants.version3) {
byte[] localEngineID =
((MPv3)snmp.getMessageProcessingModel(MessageProcessingModel.MPv3)).createLocalEngineID();
// 建立USM
USM usm = new USM(SecurityProtocols.getInstance(),
new OctetString(localEngineID), 0);
// 将USM添加至安全模式管理器中
// 安全模型管理器采用了單例模式，它内部可以維護為3個安全模型，分别對應Snmp三個版本
SecurityModels.getInstance().addSecurityModel(usm);
snmp.setLocalEngine(localEngineID, 0, 0);
// 添加使用者
snmp.getUSM().addUser(securityName，new UsmUser(securityName,authProtocol,
authPassphrase,privProtocol,privPassphrase));
}（2）同步收發消息
import org.snmp4j.*;
...
Snmp snmp = new Snmp(new DefaultUdpTransportMapping());
...
ResponseEvent response = snmp.send(requestPDU, target);
if (response.getResponse() == null) {
// request timed out
...
}else {
System.out.println("Received response from: "+
response.getPeerAddress());
// dump response PDU
System.out.println(response.getResponse().toString());
}（3）異步收發消息
import org.snmp4j.*;
import org.snmp4j.event.*;
...
Snmp snmp = new Snmp(new DefaultUdpTransportMapping());
...
// 增加監聽器
ResponseListener listener = new ResponseListener() {
public void onResponse(ResponseEvent event) {
PDU response = event.getResponse();
PDU request = event.getRequest();
if (response == null) {
System.out.println("Request "+request+" timed out");
} else {
System.out.println("Received response "+response+" on request "+
request);
}
};
snmp.sendPDU(request, target, null, listener);
...（4）實作trap
實作trap需要三步：
1）建立Snmp；
2）對于listen（）使處于網絡監聽（實際上是同于網絡程式設計中的Socket監聽）；
3）實作CommandResponder接口的監聽器，并且調用 Snmp.addCommandResponder(CommandResponder)注冊監聽器。
import org.snmp4j.*;
import org.snmp4j.smi.*;
import org.snmp4j.mp.SnmpConstants;
...
TransportMapping transport =
new DefaultUdpTransportMapping(new UdpAddress("0.0.0.0/161"));
Snmp snmp = new Snmp(transport);
if (version == SnmpConstants.version3) {
byte[] localEngineID =
((MPv3)snmp.getMessageProcessingModel(MessageProcessingModel.MPv3)).createLocalEngineID();
USM usm = new USM(SecurityProtocols.getInstance(),
new OctetString(localEngineID), 0);
SecurityModels.getInstance().addSecurityModel(usm);
snmp.setLocalEngine(localEngineID, 0, 0);
// Add the configured user to the USM
...
}
// 注冊指令響應監聽器
snmp.addCommandResponder(this);
transport.listen();
...
// 實作CommandResponder接口
public synchronized void processPdu(CommandResponderEvent e) {
PDU command = e.getPdu();
if (command != null) {
...
}
}
總結
Snmp内含了一個消息分發器，消息分發器中内含了處理網絡的線程，在使用完後最好調用close（），将其資源回歸處理。
掌握了上面所說的三個概念，基本上可以使用Snmp4j編寫Snmp的程式了。
有關Snmp4j程式設計最好也最詳細的資料：API文檔和源代碼。關于使用Snmp4j編寫Snmp程式的例子，多線程的例子可以參看源代碼 中：org.snmp4j.test包下的MultiThreadedTrapReceiver.java，完整的例子可以參看 org.snmp4j.tools.console包下的SnmpRequest（一個指令行的Snmp管理器）。
不過，要想快速和深入掌握Snmp程式設計，最好的辦法一定是先弄懂Snmp協定，這方面的資料最權威的就是RFC協定了。
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