Android Web3j OOM解決

進擊的範大叔發表於2018-07-15
AndroidWebOOM

在Android客戶端使用Web3j建立錢包、匯入錢包時都可能會產生OOM，相關issue在Github上已經有所提及：https://github.com/web3j/web3j/issues/299 。這個問題在Web3j 3.0版本以前是沒有的，由於 新版的Web3j使用spongycastle庫替換了lambdaworks庫，雖然在效率上提升了速度，但存在Android端的相容性問題。

本專案程式碼地址：https://github.com/uncleleonfan/WalletOOM.git

建立錢包OOM解決

在建立錢包時，如果建立一個Full Wallet，則會導致OOM：

public void onCreateFullWallet(View view) {
    String filePath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/full";
    File file = new File(filePath);
    file.mkdirs();
    try {
        WalletUtils.generateFullNewWalletFile("a12345678", file);
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    } catch (NoSuchProviderException e) {
        e.printStackTrace();
    } catch (InvalidAlgorithmParameterException e) {
        e.printStackTrace();
    } catch (CipherException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}
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Log如下：

"Caused by: java.lang.OutOfMemoryError: Failed to allocate a 1036 byte allocation with 16777216 free bytes and 48MB until OOM; failed due to fragmentation (required continguous free 16384 bytes for a new buffer where largest contiguous free 8192 bytes)",
"\tat org.spongycastle.util.Arrays.clone(Arrays.java:602)",
"\tat org.spongycastle.crypto.generators.SCrypt.SMix(SCrypt.java:126)",
"\tat org.spongycastle.crypto.generators.SCrypt.MFcrypt(SCrypt.java:87)",
"\tat org.spongycastle.crypto.generators.SCrypt.generate(SCrypt.java:66)",
"\tat org.web3j.crypto.Wallet.generateDerivedScryptKey(Wallet.java:136)",
"\tat org.web3j.crypto.Wallet.create(Wallet.java:74)",
"\tat org.web3j.crypto.Wallet.createStandard(Wallet.java:93)",
"\tat org.web3j.crypto.WalletUtils.generateWalletFile(WalletUtils.java:61)"
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generateFullNewWalletFile裡面會呼叫createStandard建立錢包，使用N_STANDARD，P_STANDARD來配置加密強度，直接影響需使用的記憶體大小，最終導致OOM的發生。

public static WalletFile createStandard(String password, ECKeyPair ecKeyPair)
        throws CipherException {
    return create(password, ecKeyPair, N_STANDARD, P_STANDARD);
}
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解決方法非常簡單，建立一個Light Wallet即可：

public void onCreateLightWallet(View view) {
    String filePath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/light";
    File file = new File(filePath);
    file.mkdirs();
    try {
        WalletUtils.generateLightNewWalletFile("a12345678", file);
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    } catch (NoSuchProviderException e) {
        e.printStackTrace();
    } catch (InvalidAlgorithmParameterException e) {
        e.printStackTrace();
    } catch (CipherException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

}
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generateLightNewWalletFile會呼叫createLight來建立一個輕錢包，使用N_LIGHT，P_LIGHT，他們在數值上相對較小，所以 不會OOM。

public static WalletFile createLight(String password, ECKeyPair ecKeyPair)
        throws CipherException {
    return create(password, ecKeyPair, N_LIGHT, P_LIGHT);
}
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我們可以對比一下N_STANDARD和P_STANDARD， N_LIGHT和P_LIGHT的大小：

private static final int N_LIGHT = 1 << 12;
private static final int P_LIGHT = 6;

private static final int N_STANDARD = 1 << 18;
private static final int P_STANDARD = 1;
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匯入錢包OOM解決

當我們匯入一個輕錢包時，不會產生OOM，但匯入不是一個輕錢包時，則有可能產生OOM。例如，我們使用Imtoken建立一個錢包並匯出Keystore， Keystore如下：

{"address":"9a2e2419f3af050d4730f80e7a65b9f8deb5e16f","crypto":{"cipher":"aes-128-ctr","cipherparams":{"iv":"eaccea79c27a91e307f24988186ef21a"},"ciphertext":"a163e532edf2d76beaee5c26fd2c2fab071a9cb37627aa185ac89e223e41ab97","kdf":"scrypt","kdfparams":{"dklen":32,"n":65536,"p":1,"r":8,"salt":"6a847392a029553f4152dea7bb0b6fb0ac9eec29f55e572fe94603182f5ed7f1"},"mac":"3fad2a31e18c611b10df84db9ae368ce2e189b5c35e9f111e40ca4b4bfb02491"},"id":"032c47c2-c7b7-46f8-a3f7-f526580f6f09","version":3}
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可以看到，其中n為65536，p為1，而輕錢包的n為1<<12,即2的12次方，4096，所以這不是一個輕錢包。 我們將該Keystore作為一個json檔案push到SD卡中，然後使用Web3j進行匯入：

public void onImportWallet(View view) {
    try {
        //需提前將assets目錄下的keystore.json檔案推送到手機SD裡面
        String filePath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/keystore.json";
        File file = new File(filePath);
        WalletUtils.loadCredentials("a12345678", file);
    } catch (IOException e) {
        e.printStackTrace();
    } catch (CipherException e) {
        e.printStackTrace();
    }
}
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發現同樣會OOM：

 Caused by: java.lang.OutOfMemoryError: Failed to allocate a 1036 byte allocation with 13588800 free bytes and 12MB until OOM; failed due to fragmentation (required continguous free 16384 bytes for a new buffer where largest contiguous free 12288 bytes)
    at org.spongycastle.util.Arrays.clone(Arrays.java:602)
    at org.spongycastle.crypto.generators.SCrypt.SMix(SCrypt.java:126)
    at org.spongycastle.crypto.generators.SCrypt.MFcrypt(SCrypt.java:87)
    at org.spongycastle.crypto.generators.SCrypt.generate(SCrypt.java:66)
    at org.web3j.crypto.Wallet.generateDerivedScryptKey(Wallet.java:136)
    at org.web3j.crypto.Wallet.decrypt(Wallet.java:214)
    at org.web3j.crypto.WalletUtils.loadCredentials(WalletUtils.java:112)
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通過log可以看出來，這裡和建立錢包的OOM是一樣的，都是最後呼叫generateDerivedScryptKey後導致:

private static byte[] generateDerivedScryptKey(
        byte[] password, byte[] salt, int n, int r, int p, int dkLen) throws CipherException {
    return SCrypt.generate(password, salt, n, r, p, dkLen);
}
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建立錢包可以建立一個輕錢包，匯入錢包總不能讓使用者換一個輕錢包來匯入吧。這裡，我們只能還是換回lambda庫來完成keystore的編解碼, 即我們可以自己寫一個generateDerivedScryptKey方法，將spongycastle的SCrypt換成lambda的SCrypt。我們使用MyWalletUtils和MyWallet 共同完成該任務。

public void onImportWallet(View view) {
    try {
        //需提前將assets目錄下的keystore.json檔案推送到手機SD裡面
        String filePath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/keystore.json";
        File file = new File(filePath);
        Credentials credentials = MyWalletUtils.loadCredentials("a12345678", file);
        Log.d(TAG, "address:" + credentials.getAddress());
    } catch (IOException e) {
        e.printStackTrace();
    } catch (CipherException e) {
        e.printStackTrace();
    }
}

public class MyWalletUtils {

    public static Credentials loadCredentials(String password, File source)
            throws IOException, CipherException {
        WalletFile walletFile = objectMapper.readValue(source, WalletFile.class);
        return Credentials.create(MyWallet.decrypt(password, walletFile));
    }
}

public class MyWallet {

    private static final int CURRENT_VERSION = 3;

    private static final String CIPHER = "aes-128-ctr";
    static final String AES_128_CTR = "pbkdf2";
    static final String SCRYPT = "scrypt";

    private static byte[] generateDerivedScryptKey(
            byte[] password, byte[] salt, int n, int r, int p, int dkLen)  {
        try {
            return SCrypt.scrypt(password, salt, n, r, p, dkLen);
        } catch (GeneralSecurityException e) {
            e.printStackTrace();
        }
        return null;
    }
}
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按照以上方法處理之後，就可以解決OOM，但是使用者等待的時間會稍微長一點，另外，最好還是新增一下Android平臺的libscrpt.so庫，大家可在本專案的jniLibs中找到。

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