from http://stackoverflow.com/a/18453384/1348906

Setting custom proxy in Android

public static boolean setProxy(WebView webview, String host, int port, String applicationClassName="android.app.Application") {
    // 3.2 (HC) or lower
    if (Build.VERSION.SDK_INT <= 13) {
        return setProxyUpToHC(webview, host, port);
    }
    // ICS: 4.0
    else if (Build.VERSION.SDK_INT <= 15) {
        return setProxyICS(webview, host, port);
    }
    // 4.1-4.3 (JB)
    else if (Build.VERSION.SDK_INT <= 18) {
        return setProxyJB(webview, host, port);
    }
    // 4.4 (KK)
    else {
        return setProxyKK(webview, host, port, applicationClassName);
    }
}

/**
 * Set Proxy for Android 3.2 and below.
 */
@SuppressWarnings("all")
private static boolean setProxyUpToHC(WebView webview, String host, int port) {
    Log.d(LOG_TAG, "Setting proxy with <= 3.2 API.");

    HttpHost proxyServer = new HttpHost(host, port);
    // Getting network
    Class networkClass = null;
    Object network = null;
    try {
        networkClass = Class.forName("android.webkit.Network");
        if (networkClass == null) {
            Log.e(LOG_TAG, "failed to get class for android.webkit.Network");
            return false;
        }
        Method getInstanceMethod = networkClass.getMethod("getInstance", Context.class);
        if (getInstanceMethod == null) {
            Log.e(LOG_TAG, "failed to get getInstance method");
        }
        network = getInstanceMethod.invoke(networkClass, new Object[]{webview.getContext()});
    } catch (Exception ex) {
        Log.e(LOG_TAG, "error getting network: " + ex);
        return false;
    }
    if (network == null) {
        Log.e(LOG_TAG, "error getting network: network is null");
        return false;
    }
    Object requestQueue = null;
    try {
        Field requestQueueField = networkClass
                .getDeclaredField("mRequestQueue");
        requestQueue = getFieldValueSafely(requestQueueField, network);
    } catch (Exception ex) {
        Log.e(LOG_TAG, "error getting field value");
        return false;
    }
    if (requestQueue == null) {
        Log.e(LOG_TAG, "Request queue is null");
        return false;
    }
    Field proxyHostField = null;
    try {
        Class requestQueueClass = Class.forName("android.net.http.RequestQueue");
        proxyHostField = requestQueueClass
                .getDeclaredField("mProxyHost");
    } catch (Exception ex) {
        Log.e(LOG_TAG, "error getting proxy host field");
        return false;
    }

    boolean temp = proxyHostField.isAccessible();
    try {
        proxyHostField.setAccessible(true);
        proxyHostField.set(requestQueue, proxyServer);
    } catch (Exception ex) {
        Log.e(LOG_TAG, "error setting proxy host");
    } finally {
        proxyHostField.setAccessible(temp);
    }

    Log.d(LOG_TAG, "Setting proxy with <= 3.2 API successful!");
    return true;
}

@SuppressWarnings("all")
private static boolean setProxyICS(WebView webview, String host, int port) {
    try
    {
        Log.d(LOG_TAG, "Setting proxy with 4.0 API.");

        Class jwcjb = Class.forName("android.webkit.JWebCoreJavaBridge");
        Class params[] = new Class[1];
        params[0] = Class.forName("android.net.ProxyProperties");
        Method updateProxyInstance = jwcjb.getDeclaredMethod("updateProxy", params);

        Class wv = Class.forName("android.webkit.WebView");
        Field mWebViewCoreField = wv.getDeclaredField("mWebViewCore");
        Object mWebViewCoreFieldInstance = getFieldValueSafely(mWebViewCoreField, webview);

        Class wvc = Class.forName("android.webkit.WebViewCore");
        Field mBrowserFrameField = wvc.getDeclaredField("mBrowserFrame");
        Object mBrowserFrame = getFieldValueSafely(mBrowserFrameField, mWebViewCoreFieldInstance);

        Class bf = Class.forName("android.webkit.BrowserFrame");
        Field sJavaBridgeField = bf.getDeclaredField("sJavaBridge");
        Object sJavaBridge = getFieldValueSafely(sJavaBridgeField, mBrowserFrame);

        Class ppclass = Class.forName("android.net.ProxyProperties");
        Class pparams[] = new Class[3];
        pparams[0] = String.class;
        pparams[1] = int.class;
        pparams[2] = String.class;
        Constructor ppcont = ppclass.getConstructor(pparams);

        updateProxyInstance.invoke(sJavaBridge, ppcont.newInstance(host, port, null));

        Log.d(LOG_TAG, "Setting proxy with 4.0 API successful!");
        return true;
    }
    catch (Exception ex)
    {
        Log.e(LOG_TAG, "failed to set HTTP proxy: " + ex);
        return false;
    }
}

/**
 * Set Proxy for Android 4.1 - 4.3.
 */
@SuppressWarnings("all")
private static boolean setProxyJB(WebView webview, String host, int port) {
    Log.d(LOG_TAG, "Setting proxy with 4.1 - 4.3 API.");

    try {
        Class wvcClass = Class.forName("android.webkit.WebViewClassic");
        Class wvParams[] = new Class[1];
        wvParams[0] = Class.forName("android.webkit.WebView");
        Method fromWebView = wvcClass.getDeclaredMethod("fromWebView", wvParams);
        Object webViewClassic = fromWebView.invoke(null, webview);

        Class wv = Class.forName("android.webkit.WebViewClassic");
        Field mWebViewCoreField = wv.getDeclaredField("mWebViewCore");
        Object mWebViewCoreFieldInstance = getFieldValueSafely(mWebViewCoreField, webViewClassic);

        Class wvc = Class.forName("android.webkit.WebViewCore");
        Field mBrowserFrameField = wvc.getDeclaredField("mBrowserFrame");
        Object mBrowserFrame = getFieldValueSafely(mBrowserFrameField, mWebViewCoreFieldInstance);

        Class bf = Class.forName("android.webkit.BrowserFrame");
        Field sJavaBridgeField = bf.getDeclaredField("sJavaBridge");
        Object sJavaBridge = getFieldValueSafely(sJavaBridgeField, mBrowserFrame);

        Class ppclass = Class.forName("android.net.ProxyProperties");
        Class pparams[] = new Class[3];
        pparams[0] = String.class;
        pparams[1] = int.class;
        pparams[2] = String.class;
        Constructor ppcont = ppclass.getConstructor(pparams);

        Class jwcjb = Class.forName("android.webkit.JWebCoreJavaBridge");
        Class params[] = new Class[1];
        params[0] = Class.forName("android.net.ProxyProperties");
        Method updateProxyInstance = jwcjb.getDeclaredMethod("updateProxy", params);

        updateProxyInstance.invoke(sJavaBridge, ppcont.newInstance(host, port, null));
    } catch (Exception ex) {
        Log.e(LOG_TAG,"Setting proxy with >= 4.1 API failed with error: " + ex.getMessage());
        return false;
    }

    Log.d(LOG_TAG, "Setting proxy with 4.1 - 4.3 API successful!");
    return true;
}

// from https://stackoverflow.com/questions/19979578/android-webview-set-proxy-programatically-kitkat
@SuppressLint("NewApi")
@SuppressWarnings("all")
private static boolean setProxyKK(WebView webView, String host, int port, String applicationClassName) {
    Log.d(LOG_TAG, "Setting proxy with >= 4.4 API.");

    Context appContext = webView.getContext().getApplicationContext();
    System.setProperty("http.proxyHost", host);
    System.setProperty("http.proxyPort", port + "");
    System.setProperty("https.proxyHost", host);
    System.setProperty("https.proxyPort", port + "");
    try {
        Class applictionCls = Class.forName(applicationClassName);
        Field loadedApkField = applictionCls.getField("mLoadedApk");
        loadedApkField.setAccessible(true);
        Object loadedApk = loadedApkField.get(appContext);
        Class loadedApkCls = Class.forName("android.app.LoadedApk");
        Field receiversField = loadedApkCls.getDeclaredField("mReceivers");
        receiversField.setAccessible(true);
        ArrayMap receivers = (ArrayMap) receiversField.get(loadedApk);
        for (Object receiverMap : receivers.values()) {
            for (Object rec : ((ArrayMap) receiverMap).keySet()) {
                Class clazz = rec.getClass();
                if (clazz.getName().contains("ProxyChangeListener")) {
                    Method onReceiveMethod = clazz.getDeclaredMethod("onReceive", Context.class, Intent.class);
                    Intent intent = new Intent(Proxy.PROXY_CHANGE_ACTION);

                    /*********** optional, may be need in future *************/
                    final String CLASS_NAME = "android.net.ProxyProperties";
                    Class cls = Class.forName(CLASS_NAME);
                    Constructor constructor = cls.getConstructor(String.class, Integer.TYPE, String.class);
                    constructor.setAccessible(true);
                    Object proxyProperties = constructor.newInstance(host, port, null);
                    intent.putExtra("proxy", (Parcelable) proxyProperties);
                    /*********** optional, may be need in future *************/

                    onReceiveMethod.invoke(rec, appContext, intent);
                }
            }
        }

        Log.d(LOG_TAG, "Setting proxy with >= 4.4 API successful!");
        return true;
    } catch (ClassNotFoundException e) {
        StringWriter sw = new StringWriter();
        e.printStackTrace(new PrintWriter(sw));
        String exceptionAsString = sw.toString();
        Log.v(LOG_TAG, e.getMessage());
        Log.v(LOG_TAG, exceptionAsString);
    } catch (NoSuchFieldException e) {
        StringWriter sw = new StringWriter();
        e.printStackTrace(new PrintWriter(sw));
        String exceptionAsString = sw.toString();
        Log.v(LOG_TAG, e.getMessage());
        Log.v(LOG_TAG, exceptionAsString);
    } catch (IllegalAccessException e) {
        StringWriter sw = new StringWriter();
        e.printStackTrace(new PrintWriter(sw));
        String exceptionAsString = sw.toString();
        Log.v(LOG_TAG, e.getMessage());
        Log.v(LOG_TAG, exceptionAsString);
    } catch (IllegalArgumentException e) {
        StringWriter sw = new StringWriter();
        e.printStackTrace(new PrintWriter(sw));
        String exceptionAsString = sw.toString();
        Log.v(LOG_TAG, e.getMessage());
        Log.v(LOG_TAG, exceptionAsString);
    } catch (NoSuchMethodException e) {
        StringWriter sw = new StringWriter();
        e.printStackTrace(new PrintWriter(sw));
        String exceptionAsString = sw.toString();
        Log.v(LOG_TAG, e.getMessage());
        Log.v(LOG_TAG, exceptionAsString);
    } catch (InvocationTargetException e) {
        StringWriter sw = new StringWriter();
        e.printStackTrace(new PrintWriter(sw));
        String exceptionAsString = sw.toString();
        Log.v(LOG_TAG, e.getMessage());
        Log.v(LOG_TAG, exceptionAsString);
    } catch (InstantiationException e) {
        StringWriter sw = new StringWriter();
        e.printStackTrace(new PrintWriter(sw));
        String exceptionAsString = sw.toString();
        Log.v(LOG_TAG, e.getMessage());
        Log.v(LOG_TAG, exceptionAsString);
    }
    return false;
}

private static Object getFieldValueSafely(Field field, Object classInstance) throws IllegalArgumentException, IllegalAccessException {
    boolean oldAccessibleValue = field.isAccessible();
    field.setAccessible(true);
    Object result = field.get(classInstance);
    field.setAccessible(oldAccessibleValue);
    return result;
}