types/src/main/java/com/google/common/html/types/TrustedResourceUrls.java
@GwtIncompatible("System.getEnv")
  @Nullable
  public static TrustedResourceUrl fromEnvironmentVariable(
      @CompileTimeConstant final String variableName) {
    String var = System.getenv(variableName);
    if (var == null) {
      return null;
    }
    return create(var);
  }
@GwtIncompatible("System.getEnv")
  @Nullable
  public static TrustedResourceUrl fromEnvironmentVariable(
      @CompileTimeConstant final String variableName) {
    String var = System.getenv(variableName);
    if (var == null) {
      return null;
    }
    return create(var);
  }
Creates a {@link TrustedResourceUrl} from the value of an environment variable. In a server
setting, environment variables are part of the application's deployment configuration and are
hence considered application-controlled. If the variable is not defined returns null.


<p>No runtime validation or sanitization is performed on the value of the environment variable;
being under application control, it is simply assumed to comply with the TrustedResourceUrl
contract.

@throws SecurityException  if a security manager exists and its checkPermission method doesn't
allow access to the environment variable name