UserAgent 一个熟悉又陌生的名词

背景

Http请求header中，有一串默认字符，既陌生又熟悉

这个参数我们一般很少去设置，但是okhttp给我们默认设置了okhttp/ok版本号。如果我们要恢复默认的ua，我们应该怎么办呢

Android源码层面

代码在RuntimeInit->commonInit

protected static final void commonInit() {
        if (DEBUG) Slog.d(TAG, "Entered RuntimeInit!");

        /*
         * set handlers; these apply to all threads in the VM. Apps can replace
         * the default handler, but not the pre handler.
         */
        LoggingHandler loggingHandler = new LoggingHandler();
        RuntimeHooks.setUncaughtExceptionPreHandler(loggingHandler);
        Thread.setDefaultUncaughtExceptionHandler(new KillApplicationHandler(loggingHandler));

        /*
         * Install a time zone supplier that uses the Android persistent time zone system property.
         */
        RuntimeHooks.setTimeZoneIdSupplier(() -> SystemProperties.get("persist.sys.timezone"));

        /*
         * Sets handler for java.util.logging to use Android log facilities.
         * The odd "new instance-and-then-throw-away" is a mirror of how
         * the "java.util.logging.config.class" system property works. We
         * can't use the system property here since the logger has almost
         * certainly already been initialized.
         */
        LogManager.getLogManager().reset();
        new AndroidConfig();

        /*
         * Sets the default HTTP User-Agent used by HttpURLConnection.
         */
        String userAgent = getDefaultUserAgent();
        System.setProperty("http.agent", userAgent);

        /*
         * Wire socket tagging to traffic stats.
         */
        TrafficStats.attachSocketTagger();

        initialized = true;
    }

 /**
     * Returns an HTTP user agent of the form
     * "Dalvik/1.1.0 (Linux; U; Android Eclair Build/MAIN)".
     */
    private static String getDefaultUserAgent() {
        StringBuilder result = new StringBuilder(64);
        result.append("Dalvik/");
        result.append(System.getProperty("java.vm.version")); // such as 1.1.0
        result.append(" (Linux; U; Android ");

        String version = Build.VERSION.RELEASE_OR_CODENAME; // "1.0" or "3.4b5"
        result.append(version.length() > 0 ? version : "1.0");

        // add the model for the release build
        if ("REL".equals(Build.VERSION.CODENAME)) {
            String model = Build.MODEL;
            if (model.length() > 0) {
                result.append("; ");
                result.append(model);
            }
        }
        String id = Build.ID; // "MAIN" or "M4-rc20"
        if (id.length() > 0) {
            result.append(" Build/");
            result.append(id);
        }
        result.append(")");
        return result.toString();
    }

如上分析，有两种获取方式

1、 System.getProperty("http.agent");

2、使用如上代码getDefaultUserAgent();