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            Native methods are defined in Java and written in traditional languages such as C and C++ \[[JNI 2006|AA. Bibliography#JNI 06]\]. The added extensibility comes at the cost of flexibility and portability as the code no longer conforms to the policies enforced by Java. In the past, native methods were used for performing platform specific operations, interfacing with legacy library code and improving program performance \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. Although this is no longer completely true --- because of poor portability, safetysecurity, and (quite ironically) performance issues --- native code is still used to interface with legacy code.

Defining a wrapper method facilitates installing appropriate security manager checks, performing input validation before passing the arguments to the native code or when obtaining return values, defensively copying mutable inputs and sanitizing user input. As a result every native method must be private, and must be invoked only by a wrapper method.

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In this noncompliant code example, the nativeOperation is both native and public; for that reason, untrusted callers may invoke it. Native method invocations bypass security manager checks.

The example does include a wrapper; This example includes the doOperation() wrapper method which invokes the nativeOperation() native method but fails to provide input validation or security checks.

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This compliant solution declares the native method private. Furthermore, the The doOperation() wrapper method performs routine permission checking to determine whether the succeeding operations are permitted to continue. This is followed by the creation of The method also creates a defensive copy of the mutable input array data as well as by performs range checking of the parametersarguments. The nativeOperation() method is consequently called with safe inputs. Note that the validation checks must produce outputs that conform to the input requirements of the native implementations/librariesmethods.

public final class NativeMethodWrapper {

  // private native method
  private native void nativeOperation(byte[] data, int offset, int len);

  // wrapper method performs SecurityManager and input validation checks
  public void doOperation(byte[] data, int offset, int len) {
    // permission needed to invoke native method
    securityManagerCheck();

    if (data == null) {
      throw new NullPointerException();
    }

    // copy mutable input
    data = data.clone();

    // validate input
    if ((offset < 0) || (len < 0) || (offset > (data.length - len))) {
      throw new IllegalArgumentException();
    }

    nativeOperation(data, offset, len);
  }

  static {
    // load native library in static initializer of class
    System.loadLibrary("NativeMethodLib"); 
  }
}

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