Android之ndk开发，使用ant自动创建头文件 出错

这是ant源码
<?xml version="1.0" encoding="UTF-8"?>

description

<!-- ================================= 
      target: BuildAllHeaders              
     ================================= -->
<target name="BuildAllHeaders" >
    <antcall target="BuildGetStringHeader"></antcall>
    <antcall target="BuildGetIntHeader"></antcall>
</target>

<!-- - - - - - - - - - - - - - - - - - 
      target: depends                      
     - - - - - - - - - - - - - - - - - -->
<target name="BuildGetStringHeader">
    <javah destdir="./jni" classpath="./bin/classes/" class="com.longertek.hellondk.GetString"></javah>
</target>
<!-- - - - - - - - - - - - - - - - - - 
      target: BuildGetIntHeader                      
     - - - - - - - - - - - - - - - - - -->
<target name="BuildGetIntHeader">

</target>

双击ant出现如下错误
Buildfile: E:\workspace\Android\HelloNDK\builder_headers.xml
BuildAllHeaders:
BuildGetStringHeader:
[javah] Exception in thread "main" java.lang.NullPointerException
[javah] at com.sun.tools.javah.JavahTask.run(JavahTask.java:503)
[javah] at com.sun.tools.javah.JavahTask.run(JavahTask.java:329)
[javah] at com.sun.tools.javah.Main.main(Main.java:46)

BUILD FAILED
E:\workspace\Android\HelloNDK\builder_headers.xml:19: The following error occurred while executing this line:
E:\workspace\Android\HelloNDK\builder_headers.xml:27: compilation failed

Total time: 880 milliseconds
这个错误是什么原因呢

提示编译错误，你的页面上的代码有问题。

package com.longertek.hellondk;

public class GetString {
public static native String getStr();
}

这个是我的GetString类的代码
package com.longertek.hellondk;

import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.os.Build;

public class MainActivity extends Activity {
static{
System.loadLibrary("HelloNDK");
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    TextView text = (TextView) super.findViewById(R.id.tv);
    text.setText(GetString.getStr());
}

}
这是main代码，