Protocol Buffers - Google's data interchange format
http://code.google.com/p/protobuf/
Protocol Buffers are a way of encoding structured data in an efficient yet extensible format. Google uses Protocol Buffers for almost all of its internal RPC protocols and file formats.
- Options
http://code.google.com/intl/zh-CN/apis/protocolbuffers/docs/proto.html#options
1. java_package (file option): The package you want to use for your generated Java classes.
option java_package = "com.example.foo";
2. java_outer_classname (file option): The class name for the outermost Java class (and hence the file name) you want to generate.
option java_outer_classname = "Ponycopter";
3. optimize_for (file option): Can be set to SPEED , CODE_SIZE , or LITE_RUNTIME .
option optimize_for = CODE_SIZE;
http://code.google.com/intl/zh-CN/apis/protocolbuffers/docs/reference/java-generated.html
- 根据反编译后的Java文件推导出proto文件
首先看看下面的proto文件生成的Java文件是什么样子的：
package tutorial;
option optimize_for = LITE_RUNTIME;
option java_package = "com.example.tutorial";
option java_outer_classname = "AddressBookProtos";
message Person {
required string name = 1;
required int32 id = 2;
optional string email = 3;
enum PhoneType {
MOBILE = 0;
HOME = 1;
WORK = 2;
}
message PhoneNumber {
required string number = 1;
optional PhoneType type = 2 [default = HOME];
}
repeated PhoneNumber phone = 4;
optional bytes addressBytes = 6;
}
message AddressBook {
repeated Person person = 1;
}
枚举类型会生成Java中的枚举：
public enum PhoneType
implements com.google.protobuf.Internal.EnumLite {
MOBILE(0, 0),
HOME(1, 1),
WORK(2, 2),
;
public static final int MOBILE_VALUE = 0;
public static final int HOME_VALUE = 1;
public static final int WORK_VALUE = 2;
...
}
注意同时会生成一个以_VALUE作为后缀的常量。
下面的三个域生成的Java代码是什么样子呢？
required string name = 1;
required int32 id = 2;
optional string email = 3;
首先生成一个接口PersonOrBuilder，有下面的方法：
boolean hasName();
String getName();
boolean hasId();
int getId();
boolean hasEmail();
String getEmail();
然后生成Person类，实现接口PersonOrBuilder，同时，定义了下面的常量：
public static final int NAME_FIELD_NUMBER = 1; public static final int ID_FIELD_NUMBER = 2; public static final int EMAIL_FIELD_NUMBER = 3;
可以看出，都有_FIELD_NUMBER后缀。
可以从isInitialized()函数中看判断某个域是optional还是required。optional的域不会在isInitialized()中进行检查。
public final boolean isInitialized() {
if (!hasName()) {
return false;
}
if (!hasId()) {
return false;
}
...
}
对于域是一个message而不是原始类型的情况，判断是optional还是required有些区别：
比如，对于如下的proto：
optional PhoneNumber mobile = 7; required PhoneNumber cell = 8;
对于的Java代码为：
if (!hasCell()) {
memoizedIsInitialized = 0;
return false;
}
if (hasMobile()) {
if (!getMobile().isInitialized()) {
memoizedIsInitialized = 0;
return false;
}
}
if (!getCell().isInitialized()) {
memoizedIsInitialized = 0;
return false;
}
对于required message field, Java代码中会首先用一个if语句块hasXxx()判断，然后再用一个if语句块getXxx().isInitialized()进行判断，而对于optional message field，两个if语句块会嵌套一起。
在initFields()函数中查看具体有哪些fields，再结合XXX_FIELD_NUMBER一起分析。
判断extensions：
查看registerAllExtensions()函数。
通常是使用nested extensions，需要好好理解，其实很简单，只是写法有点绕：
http://code.google.com/apis/protocolbuffers/docs/proto.html#extensions
http://code.google.com/apis/protocolbuffers/docs/reference/java-generated.html#extension
下面的例子：
public static final int MAN_FIELD_NUMBER = 123;
public static final
com.google.protobuf.GeneratedMessageLite.GeneratedExtension<
com.example.tutorial.AddressBookProtos.Person,
com.example.tutorial.AddressBookProtos.Man> man = com.google.protobuf.GeneratedMessageLite
.newSingularGeneratedExtension(
com.example.tutorial.AddressBookProtos.Person.getDefaultInstance(),
com.example.tutorial.AddressBookProtos.Man.getDefaultInstance(),
com.example.tutorial.AddressBookProtos.Man.getDefaultInstance(),
null,
123,
com.google.protobuf.WireFormat.FieldType.MESSAGE);
}
可以得知Man是Person的extension，并且field number是123.
LazyStringList对应 repeat string xxx = 1;
this change coming up in 2.4.0:
"""Added lazy conversion of UTF-8 encoded strings to String objects to improve
performance."""