protobuf原理以及实例（Varint编码）

**Protobuf 原理与实例**

Protocol Buffers（protobuf）是一种轻量级的数据序列化格式，由谷歌开发。它提供了一种高效、易用且可扩展的方式来序列化和反序列化数据。protobuf 的原理是通过定义一个 `.proto` 文件来描述数据结构，然后使用 protobuf 编译器生成相应语言的代码。

**Varint 编码**

protobuf 使用 Varint（Variable-length integer）编码来压缩整数值。这是一种高效的方式来存储小整数值。Varint 编码使用以下规则：

*0 到63 的整数值直接写入到字节流中。
*64 到16383 的整数值写入一个字节，前面有一个标志位（1），后面跟着实际的值。
*16384 到2097151 的整数值写入两个字节，前面有一个标志位（1），后面跟着实际的值。
*2097152 到268435455 的整数值写入三个字节，前面有一个标志位（1），后面跟着实际的值。

**protobuf 原理**

protobuf 的原理是通过定义一个 `.proto` 文件来描述数据结构，然后使用 protobuf 编译器生成相应语言的代码。`.proto` 文件中可以定义各种类型的字段，如整数、字符串、枚举等。

下面是一个简单的例子：

protosyntax = "proto3";

message Person {
 int32 id =1;
 string name =2;
}

bashprotoc --java_out=. person.proto

javaimport com.google.protobuf.InvalidProtocolBufferException;
import com.google.protobuf.Message;

public class Person {
 private int id;
 private String name;

 public int getId() {
 return id;
 }

 public void setId(int id) {
 this.id = id;
 }

 public String getName() {
 return name;
 }

 public void setName(String name) {
 this.name = name;
 }
}

public class PersonProto implements Message {
 private static final int ID_FIELD_NUMBER =1;
 private static final int NAME_FIELD_NUMBER =2;

 private int id_;
 private String name_;

 public static PersonProto parseFrom(com.google.protobuf.ByteString data)
 throws InvalidProtocolBufferException {
 return new PersonProto(data);
 }

 public static PersonProto parseFrom(byte[] data) throws InvalidProtocolBufferException {
 return parseFrom((com.google.protobuf.ByteString) com.google.protobuf.ByteString.wrap(data));
 }

 public static PersonProto parseFrom(com.google.protobuf.ByteString data, ExtensionRegistryLite extensionRegistry)
 throws InvalidProtocolBufferException {
 return new PersonProto(data, extensionRegistry);
 }

 public static PersonProto parseFrom(byte[] data, ExtensionRegistryLite extensionRegistry)
 throws InvalidProtocolBufferException {
 return parseFrom((com.google.protobuf.ByteString) com.google.protobuf.ByteString.wrap(data), extensionRegistry);
 }

 public static PersonProto parseFrom(com.google.protobuf.ByteString data, CodedInputStream input,
 ExtensionRegistryLite extensionRegistry) throws InvalidProtocolBufferException {
 return new PersonProto(data, input, extensionRegistry);
 }

 public static PersonProto parseFrom(CodedInputStream input) throws InvalidProtocolBufferException {
 return new PersonProto(input);
 }

 public static PersonProto parseFrom(CodedInputStream input, ExtensionRegistryLite extensionRegistry)
 throws InvalidProtocolBufferException {
 return new PersonProto(input, extensionRegistry);
 }

 private PersonProto(com.google.protobuf.ByteString data) throws InvalidProtocolBufferException {
 this(data, null, null);
 }

 private PersonProto(com.google.protobuf.ByteString data, com.google.protobuf.ExtensionRegistryLite extensionRegistry)
 throws InvalidProtocolBufferException {
 this(data, extensionRegistry, (com.google.protobuf.CodedInputStream) null);
 }

 private PersonProto(com.google.protobuf.ByteString data, com.google.protobuf.ExtensionRegistryLite extensionRegistry,
 com.google.protobuf.CodedInputStream input) throws InvalidProtocolBufferException {
 initFields();
 int tag =0;
 while (!input.isAtEnd()) {
 int wire_tag = input.readTag();
 switch (wire_tag >>3) {
 case0:
 throw new InvalidProtocolBufferException("Invalid wire type.");
 case1: // varint switch (wire_tag &7) {
 case0:
 if (ID_FIELD_NUMBER == tag) {
 id_ = input.readInt32();
 break;
 }
 tag++;
 continue;
 case1:
 if (NAME_FIELD_NUMBER == tag) {
 name_ = input.readString();
 break;
 }
 tag++;
 continue;
 }
 default: // length-delimited switch (wire_tag &7) {
 case0:
 if (ID_FIELD_NUMBER == tag) {
 id_ = input.readInt32();
 break;
 }
 tag++;
 continue;
 case1:
 if (NAME_FIELD_NUMBER == tag) {
 name_ = input.readString();
 break;
 }
 tag++;
 continue;
 }
 }
 }
 makeExtensionsImmutable();
 }

 private PersonProto(com.google.protobuf.ByteString data, com.google.protobuf.ExtensionRegistryLite extensionRegistry,
 com.google.protobuf.CodedInputStream input) throws InvalidProtocolBufferException {
 initFields();
 int tag =0;
 while (!input.isAtEnd()) {
 int wire_tag = input.readTag();
 switch (wire_tag >>3) {
 case0:
 throw new InvalidProtocolBufferException("Invalid wire type.");
 case1: // varint switch (wire_tag &7) {
 case0:
 if (ID_FIELD_NUMBER == tag) {
 id_ = input.readInt32();
 break;
 }
 tag++;
 continue;
 case1:
 if (NAME_FIELD_NUMBER == tag) {
 name_ = input.readString();
 break;
 }
 tag++;
 continue;
 }
 default: // length-delimited switch (wire_tag &7) {
 case0:
 if (ID_FIELD_NUMBER == tag) {
 id_ = input.readInt32();
 break;
 }
 tag++;
 continue;
 case1:
 if (NAME_FIELD_NUMBER == tag) {
 name_ = input.readString();
 break;
 }
 tag++;
 continue;
 }
 }
 }
 makeExtensionsImmutable();
 }

 private PersonProto(com.google.protobuf.ByteString data, com.google.protobuf.ExtensionRegistryLite extensionRegistry)
 throws InvalidProtocolBufferException {
 initFields();
 int tag =0;
 while (!input.isAtEnd()) {
 int wire_tag = input.readTag();
 switch (wire_tag >>3) {
 case0:
 throw new InvalidProtocolBufferException("Invalid wire type.");
 case1: // varint switch (wire_tag &7) {
 case0:
 if (ID_FIELD_NUMBER == tag) {
 id_ = input.readInt32();
 break;
 }
 tag++;
 continue;
 case1:
 if (NAME_FIELD_NUMBER == tag) {
 name_ = input.readString();
 break;
 }
 tag++;
 continue;
 }
 default: // length-delimited switch (wire_tag &7) {
 case0:
 if (ID_FIELD_NUMBER == tag) {
 id_ = input.readInt32();
 break;
 }
 tag++;
 continue;
 case1:
 if (NAME_FIELD_NUMBER == tag) {
 name_ = input.readString();
 break;
 }
 tag++;
 continue;
 }
 }
 }
 makeExtensionsImmutable();
 }

 private PersonProto(com.google.protobuf.ByteString data, com.google.protobuf.ExtensionRegistryLite extensionRegistry,
 com.google.protobuf.CodedInputStream input) throws InvalidProtocolBufferException {
 initFields();
 int tag =0;
 while (!input.isAtEnd()) {
 int wire_tag = input.readTag();
 switch (wire_tag >>3) {
 case0:
 throw new InvalidProtocolBufferException("Invalid wire type.");
 case1: // varint switch (wire_tag &7) {
 case0:
 if (ID_FIELD_NUMBER == tag) {
 id_ = input.readInt32();
 break;
 }
 tag++;
 continue;
 case1:
 if (NAME_FIELD_NUMBER == tag) {
 name_ = input.readString();
 break;
 }
 tag++;
 continue;
 }
 default: // length-delimited switch (wire_tag &7) {
 case0:
 if (ID_FIELD_NUMBER == tag) {
 id_ =