在Android系统中有几个版本号经常遇到，有时还要做特殊处理。下面整理一下，这些都保留在文件系统的/system/build.prop文件里，build.prop相当于Windows下的注册表，这个文件内定义了系统初始（或永久）的一些参数属性、功能的开放等。

下面的代码均是基于Android8.1分析。

系统设置--关于手机--版本号

1、在系统设置代码中搜索“版本号”，AndroidStudio中全局搜索快捷键“Ctrl+Shift+f”。

<string name="build_number" msgid="3075795840572241758">"版本号"</string>
全局搜索“build_number”，找到com.android.settings.deviceinfo.BuildNumberPreferenceController类。

private static final String KEY_BUILD_NUMBER = "build_number";

     public void displayPreference(PreferenceScreen screen) {
        super.displayPreference(screen);
        final Preference preference = screen.findPreference(KEY_BUILD_NUMBER);
        if (preference != null) {
            try {
                preference.setSummary(BidiFormatter.getInstance().unicodeWrap(Build.DISPLAY));
                preference.setEnabled(true);
            } catch (Exception e) {
                preference.setSummary(R.string.device_info_default);
            }
        }
    }

此时关键就是找到Build.DISPLAY。

这里是定义在android.os.Build类中，该类位于frameworks/base/core/java目录下，如下：

/** A build ID string meant for displaying to the user */
    public static final String DISPLAY = getString("ro.build.display.id");
    private static String getString(String property) {
        return SystemProperties.get(property, UNKNOWN);
    }

我们看到了DISPLAY是Build类中的静态变量，表示“ro.build.display.id”的属性值。

2、此时我们就需要知道“ro.build.display.id”属性值是在哪里定义的。

这里我们在Android源码环境下全局搜索“ro.build.display.id”，命令：grep -r "ro.build.display.id" ./

其实只在build目录下搜索即可，build存放系统编译规则。

在build/make/tools/buildinfo.sh文件中搜索出：

echo "ro.build.display.id=$BUILD_DISPLAY_ID"

我们继续在build目录下，搜索“BUILD_DISPLAY_ID”，命令：grep -r "BUILD_DISPLAY_ID"。在文件build/make/core/MakeFile中搜索出：

# Display parameters shown under Settings -> About Phone
ifeq ($(TARGET_BUILD_VARIANT),user)
  # User builds should show:
  # release build number or branch.buld_number non-release builds
  # Dev. branches should have DISPLAY_BUILD_NUMBER set
  ifeq (true,$(DISPLAY_BUILD_NUMBER))
    BUILD_DISPLAY_ID := $(BUILD_ID).$(BUILD_NUMBER_FROM_FILE) $(BUILD_KEYS)
  else
    BUILD_DISPLAY_ID := $(BUILD_ID) $(BUILD_KEYS)
  endif
else
  # Non-user builds should show detailed build information
  BUILD_DISPLAY_ID := $(build_desc)
endif

BUILD_ID 定义在build/make/core/build_id.mk中，如下：

# BUILD_ID is usually used to specify the branch name
# (like "MAIN") or a branch name and a release candidate
# (like "CRB01").  It must be a single word, and is
# capitalized by convention.
 
export BUILD_ID=OPM1.171019.011

BUILD_NUMBER_FROM_FILE 定义在build/make/core/main.mk中，如下：

BUILD_NUMBER_FROM_FILE := $$(cat $(OUT_DIR)/build_number.txt)

这里OUT_DIR就是out目录， build_number.txt就是在out目录下生成的一个文件，内容如下：

eng.sgf.20180421.152052

这里是eng版本。

BUILD_KEYS定义在build/make/core/MakeFile中，表示系统签名，test-keys只适用于开发阶段。如下：

# The "test-keys" tag marks builds signed with the old test keys,
# which are available in the SDK.  "dev-keys" marks builds signed with
# non-default dev keys (usually private keys from a vendor directory).
# Both of these tags will be removed and replaced with "release-keys"
# when the target-files is signed in a post-build step.
ifeq ($(DEFAULT_SYSTEM_DEV_CERTIFICATE),build/target/product/security/testkey)
BUILD_KEYS := test-keys
else
BUILD_KEYS := dev-keys
endif
如果不是user版本，BUILD_DISPLAY_ID的取值就是$(build_desc)
# A human-readable string that descibes this build in detail.
build_desc := $(TARGET_PRODUCT)-$(TARGET_BUILD_VARIANT) $(PLATFORM_VERSION) $(BUILD_ID) $(BUILD_NUMBER_FROM_FILE) $(BUILD_VERSION_TAGS)

BUILD_NUMBER定义在build/make/core/version_default.mk文件中，如下：

ifndef BUILD_NUMBER
  # BUILD_NUMBER should be set to the source control value that
  # represents the current state of the source code.  E.g., a
  # perforce changelist number or a git hash.  Can be an arbitrary string
  # (to allow for source control that uses something other than numbers),
  # but must be a single word and a valid file name.
  #
  # If no BUILD_NUMBER is set, create a useful "I am an engineering build
  # from this date/time" value.  Make it start with a non-digit so that
  # anyone trying to parse it as an integer will probably get "0".
  BUILD_NUMBER := eng.$(shell echo $${USER:0:6}).$(shell $(DATE) +%Y%m%d.%H%M%S)
endif
总结：想要修改版本号，可以通过两个部分修改。
1、修改Java文件，直接写死；
2、修改编译系统，在device中修改。