com.sun.jarsigner.ContentSigner is a JDK specific API that existed in JDK 8. It used to be in tools.jar. It is currently in the jdk.jartool module (com.sun.jarsigner is an exported package). It's not currently possible to compile code using ContentSigner with --release 8.

How to reproduce:

c $ cat X.java
import com.sun.jarsigner.*;
public class X extends ContentSigner {
    @Override
    public byte[] generateSignedData(ContentSignerParameters parameters,
            boolean omitContent, boolean applyTimestamp) {
        return "1234".getBytes();
    }
}
c $ javac --release 8 X.java
X.java:2: error: cannot find symbol
public class X extends ContentSigner {
                       ^
  symbol: class ContentSigner
X.java:4: error: cannot find symbol
    public byte[] generateSignedData(ContentSignerParameters parameters,
                                     ^
  symbol: class ContentSignerParameters
  location: class X
X.java:1: error: package com.sun.jarsigner does not exist
import com.sun.jarsigner.*;
^
X.java:3: error: method does not override or implement a method from a supertype
    @Override
    ^
4 errors