1.Android 7.1之前，系统签名如下：

 java -jar signapk.jar platform.x509.pem platform.pk8 source.apk target.apk

2.Android 7.1及以后，系统签名会出错：

Exception in thread "main" java.lang.UnsatisfiedLinkError: no conscrypt_openjdk_jni in java.library.path
        at java.lang.ClassLoader.loadLibrary(Unknown Source)
        at java.lang.Runtime.loadLibrary0(Unknown Source)
        at java.lang.System.loadLibrary(Unknown Source)
        at org.conscrypt.NativeCryptoJni.init(NativeCryptoJni.java:25)
        at org.conscrypt.NativeCrypto.<clinit>(NativeCrypto.java:54)
        at org.conscrypt.OpenSSLBIOInputStream.<init>(OpenSSLBIOInputStream.java:34)
        at org.conscrypt.OpenSSLX509Certificate.fromX509PemInputStream(OpenSSLX509Certificate.java:119)
        at org.conscrypt.OpenSSLX509CertificateFactory$1.fromX509PemInputStream(OpenSSLX509CertificateFactory.java:220)
        at org.conscrypt.OpenSSLX509CertificateFactory$1.fromX509PemInputStream(OpenSSLX509CertificateFactory.java:216)
        at org.conscrypt.OpenSSLX509CertificateFactory$Parser.generateItem(OpenSSLX509CertificateFactory.java:94)
        at org.conscrypt.OpenSSLX509CertificateFactory.engineGenerateCertificate(OpenSSLX509CertificateFactory.java:272)
        at java.security.cert.CertificateFactory.generateCertificate(Unknown Source)
        at com.android.signapk.SignApk.readPublicKey(SignApk.java:182)
        at com.android.signapk.SignApk.main(SignApk.java:1087)

3.Linux系统

    找打prebuilts/sdk/tools/linux/lib64/libconscrypt_openjdk_jni.so，

    文件夹命名为sign,将platform.x509.pem ，platform.pk8，signapk.jar，libconscrypt_openjdk_jni.so，放到一起，

    在linux系统里执行：

    java -Djava.library.path=. -jar signapk.jar platform.x509.pem platform.pk8 source.apk target.apk