将字符串转换为 JNA 的指针

Question

我正在尝试使用 JNA 在 Windows 中查询文件的有效权限。最终，我计划使用GetEffectiveRightsFromAcl function，但要这样做，我需要提供一个指向已填充TRUSTEE structure 的指针。 JNA 平台（platform.jar）似乎没有定义这个结构，所以我试图自己定义它。这是我目前所拥有的：

public static class TRUSTEE extends Structure {
    public TRUSTEE() {
        super();
    }
    public TRUSTEE(Pointer p) {
        super(p);
        read();
    }

    public Pointer pMultipleTrustee;
    public int MultipleTrusteeOperation;
    public int TrusteeForm;
    public int TrusteeType;
    public Pointer ptstrName;
}

我正在尝试填充这样的结构：

private TRUSTEE createTrusteeForCurrentUser() {
    TRUSTEE result = new TRUSTEE();
    result.TrusteeForm = TRUSTEE_FORM.TRUSTEE_IS_NAME;
    result.TrusteeType = TRUSTEE_TYPE.TRUSTEE_IS_USER;

    String strName = "CURRENT_USER";
    // How can I set result.ptstrName using strName?
}

This Google Groups thread 建议在调用 char * 时在结构中使用 String 字段。但是，我认为这不适合我的情况，考虑到 ptstrName 字段可以指向不同类型的事物，具体取决于 TrusteeForm 的值。所以，我想我需要以某种方式从String 转换为Pointer。我在 JNA 中找到了 NativeString 类，它可以工作，除了它是一个包私有类。

将Java String 转换为本机格式并获得Pointer 的推荐方法是什么？我是否为TRUSTEE 结构使用了正确的数据类型？我对 JNA 有点陌生，所以如果我遗漏了一些明显的东西，请见谅。

更新

我找到了解决问题的方法，但如果有人有更好的解决方案，我仍然想听听。

Answer 1

尝试在http://jna.java.net/javadoc/com/sun/jna/Pointer.html 中使用指针类。

Answer 2

我通过复制 package-private NativeString 类的源代码并在我的项目中创建一个公共副本解决了这个问题。由于在构造函数中使用了包私有方法，我不得不做一个小改动。

更新：正如@fragorl 在 cmets 中指出的那样，下面显示的 NativeString 的实现现在已经过时了。

---

用法：

private static TRUSTEE createTrusteeForCurrentUser() {
    TRUSTEE result = new TRUSTEE();
    result.TrusteeForm = TRUSTEE_FORM.TRUSTEE_IS_NAME;
    result.TrusteeType = TRUSTEE_TYPE.TRUSTEE_IS_USER;
    result.ptstrName = new NativeString("CURRENT_USER",true).getPointer();
    result.write();
    return result;
}

NativeString.java：

/** Provides a temporary allocation of an immutable C string 
 * (<code>const char*</code> or <code>const wchar_t*</code>) for use when 
 * converting a Java String into a native memory function argument.  
 *
 * @author  Todd Fast, todd.fast@sun.com
 * @author twall@users.sf.net
 */
public class NativeString implements CharSequence, Comparable {

    private Pointer pointer;
    private boolean wide;

    /** Create a native string (NUL-terminated array of <code>char</code>).<p>
     * If the system property <code>jna.encoding</code> is set, its value will
     * be used to encode the native string.  If not set or if the encoding
     * is unavailable, the default platform encoding will be used. 
     */
    public NativeString(String string) {
        this(string, false);
    }

    /** Create a native string as a NUL-terminated array of <code>wchar_t</code>
     * (if <code>wide</code> is true) or <code>char</code>.<p>
     * If the system property <code>jna.encoding</code> is set, its value will
     * be used to encode the native <code>char</code>string.  
     * If not set or if the encoding is unavailable, the default platform 
     * encoding will be used. 
     * 
     * @param string value to write to native memory
     * @param wide whether to store the String as <code>wchar_t</code>
     */
    public NativeString(String string, boolean wide) {
        if (string == null) {
            throw new NullPointerException("String must not be null");
        }
        // Allocate the memory to hold the string.  Note, we have to
        // make this 1 element longer in order to accommodate the terminating 
        // NUL (which is generated in Pointer.setString()).
        this.wide = wide;
        if (wide) {
            int len = (string.length() + 1 ) * Native.WCHAR_SIZE;
            pointer = new Memory(len);
            pointer.setString(0, string, true);
        }
        else {
            byte[] data = Native.toByteArray(string);
            pointer = new Memory(data.length + 1);
            pointer.write(0, data, 0, data.length);
            pointer.setByte(data.length, (byte)0);
        }
    }

    public int hashCode() {
        return toString().hashCode();
    }

    public boolean equals(Object other) {

        if (other instanceof CharSequence) {
            return compareTo(other) == 0;
        }
        return false;
    }

    public String toString() {
        String s = wide ? "const wchar_t*" : "const char*";
        s += "(" + pointer.getString(0, wide) + ")";
        return s;
    }

    public Pointer getPointer() {
        return pointer;
    }

    public char charAt(int index) {
        return toString().charAt(index);
    }

    public int length() {
        return toString().length();
    }

    public CharSequence subSequence(int start, int end) {
        return CharBuffer.wrap(toString()).subSequence(start, end);
    }

    public int compareTo(Object other) {

        if (other == null)
            return 1;

        return toString().compareTo(other.toString());
    }
}

Answer 3

假设您希望在本机端使用 char *（如果字符串包含非 ascii 字符，您可能需要分配更多内存），

String myString = "CURRENT_USER";
Pointer m = new Memory(myString.length() + 1); // WARNING: assumes ascii-only string
m.setString(0, myString); 

然后，您可以在需要引用“本机”字符串的任何地方使用m。

对于宽字符串 (wchar_t *)，

String myString = "CURRENT_USER";
Pointer m = new Memory(Native.WCHAR_SIZE * (myString.length() + 1));
m.setWideString(0, myString);