在網絡傳輸時，傳輸的是二進制數據，所以發送端需要將序列化對象轉變為二進制數據，也就是序列化過程。接收端需要將二進制數據轉化為序列化對象，也就是反序列化過程。在序列化和反序列化過程中，需要定義一種對數據相互轉變的一致性協議，也就是序列化協議。zookeeper使用Jute作為序列化組件。首先看下Jute的使用：
public class RequestHeader implements Record {
private int xid;
private int type;
public RequestHeader() {
}
public RequestHeader(
int xid,
int type) {
this.xid=xid;
this.type=type;
}
public void serialize(OutputArchive a_, String tag) throws java.io.IOException {
a_.startRecord(this,tag);
//先寫入xid
a_.writeInt(xid,"xid");
a_.writeInt(type,"type");
a_.endRecord(this,tag);
}
public void deserialize(InputArchive a_, String tag) throws java.io.IOException {
a_.startRecord(tag);
//先讀出xid
xid=a_.readInt("xid");
type=a_.readInt("type");
a_.endRecord(tag);
}
//測試方法
public void testJute() throws IOException {
//實現Record接口，自定義序列化
RequestHeader requestHeader = new RequestHeader(1, ZooDefs.OpCode.create);
System.out.print("requestHeader: " +requestHeader );
//序列化
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
BinaryOutputArchive binaryOutputArchive = BinaryOutputArchive.getArchive(outputStream);
requestHeader.serialize(binaryOutputArchive,"header");
//通常是TCP網絡通信對象
ByteBuffer bb = ByteBuffer.wrap(outputStream.toByteArray());
//反序列化
RequestHeader requestHeader1 = new RequestHeader();
ByteBufferInputStream inputStream = new ByteBufferInputStream(bb);
BinaryInputArchive binaryInputArchive = BinaryInputArchive.getArchive(inputStream);
requestHeader1.deserialize(binaryInputArchive,"header");
System.out.print("requestHeader1: " + requestHeader1);
outputStream.close();
inputStream.close();
}
1.定義序列化對象RequestHeader，需要實現Record接口的serialize和deserialize接口
2.構建序列化器BinaryOutputArchive，調用serialize方法將對象序列化流中
3.構建反序列化器BinaryInputArchive，調用deserialize方法將流反序列化為對象
從上面的使用我們可以看出，對RequestHeader對象的序列化 就是對其成員變量xid，type的按順序的寫入序列化器BinaryOutputArchive，反序列化就是從反序列化器BinaryInputArchive按順序的讀出xid，type。
所以序列化組件Jute的實現關鍵就是對序列化對象，序列化器和反序列化器的設計。
序列化對象
所有的序列化對象都要實現Record接口，它定義了serialize和deserialize方法用于子類自己實現自己的序列化和反序列方式。
public interface Record {
public void serialize(OutputArchive archive, String tag)
throws IOException;
public void deserialize(InputArchive archive, String tag)
throws IOException;
}
zookeeper的org.apache.zookeeper.proto包下定義了很多用于網絡通信和數據存儲所需要的序列化對象。
序列化器
在zookeeper中序列化就是將Record對象變為二進制數據的過程，序列化器接口為OutputArchive
public interface OutputArchive {
public void writeByte(byte b, String tag) throws IOException;
public void writeBool(boolean b, String tag) throws IOException;
public void writeInt(int i, String tag) throws IOException;
public void writeLong(long l, String tag) throws IOException;
public void writeFloat(float f, String tag) throws IOException;
public void writeDouble(double d, String tag) throws IOException;
public void writeString(String s, String tag) throws IOException;
public void writeBuffer(byte buf[], String tag)
throws IOException;
public void writeRecord(Record r, String tag) throws IOException;
public void startRecord(Record r, String tag) throws IOException;
public void endRecord(Record r, String tag) throws IOException;
public void startVector(List<?> v, String tag) throws IOException;
public void endVector(List<?> v, String tag) throws IOException;
public void startMap(TreeMap<?,?> v, String tag) throws IOException;
public void endMap(TreeMap<?,?> v, String tag) throws IOException;
}
有三種實現：BinaryOutputArchive，CsvOutputArchive和XmlOutputArchive，分別對應無特殊格式，有csv格式和有xml格式的數據序列化。
BinaryOutputArchive
public class BinaryOutputArchive implements OutputArchive {
private ByteBuffer bb = ByteBuffer.allocate(1024);
private DataOutput out;
public static BinaryOutputArchive getArchive(OutputStream strm) {
return new BinaryOutputArchive(new DataOutputStream(strm));
}
/** Creates a new instance of BinaryOutputArchive */
public BinaryOutputArchive(DataOutput out) {
this.out = out;
}
public void writeByte(byte b, String tag) throws IOException {
out.writeByte(b);
}
public void writeBool(boolean b, String tag) throws IOException {
out.writeBoolean(b);
}
··········省略代碼·····
}
可以看到BinaryOutputArchive其實是對DataOutput out的包裝，從而實現了對各種數據類型的寫入，tag并不會寫入進二進制數據中，而對于CsvOutputArchive和XmlOutputArchive，如果想在二進制數據中保存對應格式，就需要tag控制，如XmlOutputArchive中
public void writeBool(boolean b, String tag) throws IOException {
printBeginEnvelope(tag);
stream.print("<boolean>");
stream.print(b ? "1" : "0");
stream.print("</boolean>");
printEndEnvelope(tag);
}
private void printBeginEnvelope(String tag) {
if (!compoundStack.empty()) {
String s = compoundStack.peek();
if ("struct".equals(s)) {
putIndent();
stream.print("<member>\n");
addIndent();
putIndent();
stream.print("<name>"+tag+"</name>\n");
putIndent();
stream.print("<value>");
} else if ("vector".equals(s)) {
stream.print("<value>");
} else if ("map".equals(s)) {
stream.print("<value>");
}
} else {
stream.print("<value>");
}
}
反序列化器
在zookeeper中反序列化就是將二進制數據變為Record對象的過程，反序列化器接口為InputArchive
public interface InputArchive {
public byte readByte(String tag) throws IOException;
public boolean readBool(String tag) throws IOException;
public int readInt(String tag) throws IOException;
public long readLong(String tag) throws IOException;
public float readFloat(String tag) throws IOException;
public double readDouble(String tag) throws IOException;
public String readString(String tag) throws IOException;
public byte[] readBuffer(String tag) throws IOException;
public void readRecord(Record r, String tag) throws IOException;
public void startRecord(String tag) throws IOException;
public void endRecord(String tag) throws IOException;
public Index startVector(String tag) throws IOException;
public void endVector(String tag) throws IOException;
public Index startMap(String tag) throws IOException;
public void endMap(String tag) throws IOException;
}
同樣有三種實現：BinaryInputArchive，CsvInputArchive和XmlInputArchive，分別對應無特殊格式，有csv格式和有xml格式的數據序列化。
public class BinaryInputArchive implements InputArchive {
private DataInput in;
static public BinaryInputArchive getArchive(InputStream strm) {
return new BinaryInputArchive(new DataInputStream(strm));
}
/** Creates a new instance of BinaryInputArchive */
public BinaryInputArchive(DataInput in) {
this.in = in;
}
public byte readByte(String tag) throws IOException {
return in.readByte();
}
public boolean readBool(String tag) throws IOException {
return in.readBoolean();
}
}
··········省略代碼·····
}
可以看到BinaryInputArchive其實是對DataInput in的包裝，從而實現了對各種數據類型的讀取，tag并不會寫入進二進制數據中。
實際zookeeper的客戶端在向服務端發送請求時，通信協議體如下：
len為請求數據的總長度，占4位。請求頭就是事例中的RequestHeader的xid和type。xid用于記錄客戶端請求發起的先后順序，占4位。type代表請求的操作類型，占4位。這樣子在服務端反序列化時，就可以根據type的值來選擇對應的Record來讀取請求體內容。
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