如何通過 cgo/JNA 傳遞 Java 字符串數組 []String to Go

Question

我想通過 JNA 將 Java 中的 String[] 傳遞給我的 Go 函數。

我的 go 函數具有以下簽名：

func PredicateEval(keys, values []string, expression string) *C.char

我已將鏈接模式的 go 庫編譯為“c-shared”。 我在 Java 中有一個GoString定義為：

package predicates;
import com.ochafik.lang.jnaerator.runtime.NativeSize;
import com.sun.jna.Pointer;
import com.sun.jna.Structure;
import java.util.Arrays;
import java.util.List;
/**
 * <i>native declaration : coverage_server/predicate_jvm_bridge/lib/libtest.h</i><br>
 * This file was autogenerated by <a href="http://jnaerator.googlecode.com/">JNAerator</a>,<br>
 * a tool written by <a href="http://ochafik.com/">Olivier Chafik</a> that <a href="http://code.google.com/p/jnaerator/wiki/CreditsAndLicense">uses a few opensource projects.</a>.<br>
 * For help, please visit <a href="http://nativelibs4java.googlecode.com/">NativeLibs4Java</a> , <a href="http://rococoa.dev.java.net/">Rococoa</a>, or <a href="http://jna.dev.java.net/">JNA</a>.
 */
public class _GoString_ extends Structure {
    /** C type : const char* */
    public Pointer p;
    public NativeSize n;
    public _GoString_() {
        super();
    }
    protected List<String> getFieldOrder() {
        return Arrays.asList("p", "n");
    }
    /** @param p C type : const char* */
    public _GoString_(Pointer p, NativeSize n) {
        super();
        this.p = p;
        this.n = n;
    }
    public static class ByReference extends _GoString_ implements Structure.ByReference {
        
    };
    public static class ByValue extends _GoString_ implements Structure.ByValue {
        
    };
}

我還有一個 Go 切片定義為：

package predicates;
import com.sun.jna.Pointer;
import com.sun.jna.Structure;
import java.util.Arrays;
import java.util.List;
/**
 * <i>native declaration : coverage_server/predicate_jvm_bridge/lib/libtest.h</i><br>
 * This file was autogenerated by <a href="http://jnaerator.googlecode.com/">JNAerator</a>,<br>
 * a tool written by <a href="http://ochafik.com/">Olivier Chafik</a> that <a href="http://code.google.com/p/jnaerator/wiki/CreditsAndLicense">uses a few opensource projects.</a>.<br>
 * For help, please visit <a href="http://nativelibs4java.googlecode.com/">NativeLibs4Java</a> , <a href="http://rococoa.dev.java.net/">Rococoa</a>, or <a href="http://jna.dev.java.net/">JNA</a>.
 */
public class GoSlice extends Structure {
    /** C type : void* */
    public Pointer data;
    /** C type : GoInt */
    public long len;
    /** C type : GoInt */
    public long cap;
    public GoSlice() {
        super();
    }
    protected List<String> getFieldOrder() {
        return Arrays.asList("data", "len", "cap");
    }
    /**
     * @param data C type : void*<br>
     * @param len C type : GoInt<br>
     * @param cap C type : GoInt
     */
    public GoSlice(Pointer data, long len, long cap) {
        super();
        this.data = data;
        this.len = len;
        this.cap = cap;
    }
    public static class ByReference extends GoSlice implements Structure.ByReference {
        
    };
    public static class ByValue extends GoSlice implements Structure.ByValue {
        
    };
}

這是我嘗試將 Java []String 轉換為 Go string[]。

  static {
    try {
      Field field = sun.misc.Unsafe.class.getDeclaredField("theUnsafe");
      field.setAccessible(true);
      unsafe = (sun.misc.Unsafe) field.get(null);
      Class<?> clazz = ByteBuffer.allocateDirect(0).getClass();
      DIRECT_BYTE_BUFFER_ADDRESS_OFFSET = unsafe.objectFieldOffset(Buffer.class.getDeclaredField("address"));
      DIRECT_BYTE_BUFFER_CLASS = clazz;
    } catch (Exception e) {
      throw new AssertionError(e);
    }
  }

  private static long getAddress(ByteBuffer buffer) {
    assert buffer.getClass() == DIRECT_BYTE_BUFFER_CLASS;
    return unsafe.getLong(buffer, DIRECT_BYTE_BUFFER_ADDRESS_OFFSET);
  }

  public static _GoString_.ByValue JavaStringToGo(String jstr) {
    try {
      byte[] bytes = jstr.getBytes("utf-8");
      //ByteBuffer bb = ByteBuffer.wrap(bytes);
      ByteBuffer bb = ByteBuffer.allocateDirect(bytes.length);
      bb.put(bytes);
      Pointer p = new Pointer(getAddress(bb));
      _GoString_.ByValue value = new _GoString_.ByValue();

      value.n = new NativeSize(bytes.length);
      value.p = p;
      return value;
    } catch (UnsupportedEncodingException e) {
      throw new RuntimeException(e);
    }
  }
  
  public static GoSlice.ByValue JavaStringArrayToGoStringSlice(String[] strings) {
    _GoString_.ByValue[] goStrings = new _GoString_.ByValue[strings.length];
    for (int i = 0; i < strings.length; i++) {
      goStrings[i] = JavaStringToGo(strings[i]);
    }

    Memory arr = new Memory(strings.length * Native.getNativeSize(_GoString_.class));
    for (int i = 0; i < strings.length; i++) {
      System.out.println(Native.getNativeSize(_GoString_.class));
      byte[] bytes = goStrings[0].getPointer().getByteArray(0, Native.getNativeSize(_GoString_.class));
      arr.write(i*Native.getNativeSize(_GoString_.class), bytes, 0, bytes.length);
    }
    GoSlice.ByValue slice = new GoSlice.ByValue();
    slice.data = arr;
    slice.len = strings.length;
    slice.cap = strings.length;

    return slice;
  }

一切都可以編譯，但是當我嘗試訪問 Go 端的切片元素時，出現了段錯誤：

unexpected fault address 0xb01dfacedebac1e
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb01dfacedebac1e pc=0x10d7d3d6f]

goroutine 17 [running, locked to thread]:

Answer 1

您正在失去對實際字符串的內存分配的跟蹤（和控制）。

您對_GoString_映射僅包括指針（4 或 8 字節）和NativeSize （4 或 8 字節size_t ）的分配。 此映射假定Pointer保持有效：

public class _GoString_ extends Structure {
    /** C type : const char* */
    public Pointer p;
    public NativeSize n;
    // constructors, etc.
}

但是，當您將值分配給p您只會跟蹤指針的地址，而不是實際的內存分配（我已在代碼中添加了注釋）：

  public static _GoString_.ByValue JavaStringToGo(String jstr) {
    try {
      byte[] bytes = jstr.getBytes("utf-8");
      // 
      // Here you allocate memory for the bytes
      //
      ByteBuffer bb = ByteBuffer.allocateDirect(bytes.length);
      bb.put(bytes);
      // 
      // Here you only keep track of the pointer to the bytes
      //
      Pointer p = new Pointer(getAddress(bb));
      // 
      // You never reference bb again, it is no longer reachable
      // and its allocation can be reclaimed by the system
      //
      _GoString_.ByValue value = new _GoString_.ByValue();

      value.n = new NativeSize(bytes.length);
      value.p = p;
      return value;
    } catch (UnsupportedEncodingException e) {
      throw new RuntimeException(e);
    }
  }

作為直接字節緩沖區，內存位置和（解除）分配機制與普通對象和 GC 不同，但基本原則適用，一旦您失去對 Java 對象（ ByteBuffer ）的引用，您將無法控制該本地對象何時內存將被釋放。 （當bb被 GC 處理時，其內部字段包括在處理時將觸發釋放的引用。）

一種可能的解決方案是向您的_GoString_類添加一個private字段，該字段維護對ByteBuffer的強引用並防止系統回收其內存（可能添加一個ByteBuffer構造器）。

另一種解決方案是將 JNA 的Memory類用於字符串，並將該Memory對象（擴展Pointer ）直接存儲到p字段。 我不確定您為什么為此應用程序選擇了直接字節緩沖區，因此這可能不適用於您的用例，但它肯定會簡化您的代碼。