如何从JVM外部地址获取String？从LongBuffer地址转char*为byte[]可行吗？

Absolutely—this is possible, but it requires working with Java's low-level native/unsafe APIs since you're directly interacting with off-heap memory pointers. Let's walk through the options and critical caveats to do this safely:

Key Background First #

First, confirm your LongBuffer holds a valid native memory address pointing to a char* within your process. If this address is invalid (e.g., already freed, from another process), any attempt to access it will crash the JVM immediately—so double-check validity before proceeding.

Option 1: Use sun.misc.Unsafe (or JDK 9+ jdk.internal.misc.Unsafe) #

This is a quick way to prototype, but note that Unsafe is an internal, unsupported API that may change or be removed in future JDK versions.

Steps to implement:

• Retrieve the Unsafe instance via reflection (it’s not publicly accessible):

  private static Unsafe getUnsafe() throws Exception {
      Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
      unsafeField.setAccessible(true);
      return (Unsafe) unsafeField.get(null);
  }
  

• Extract the pointer value from your LongBuffer:

  long charPtr = yourLongBuffer.get(0); // Assuming the address is stored at index 0
  

• Define the total byte length of your char* data (you need to know this upfront):
  • If your C char is 1-byte (ASCII/UTF-8), total bytes = number of chars
  • If it’s a 2-byte wide character (like Windows wchar_t), total bytes = number of chars × 2
• Allocate a Java byte[] and copy the native memory into it:

  Unsafe unsafe = getUnsafe();
  int totalBytes = charCount * sizeofChar; // Replace with actual byte count
  byte[] result = new byte[totalBytes];
  // Copy from native pointer to Java byte array
  unsafe.copyMemory(null, charPtr, result, Unsafe.ARRAY_BYTE_BASE_OFFSET, totalBytes);
  

Option 2: Use JNI (Recommended for Production) #

JNI is the official, supported way to interact with native code, making it more stable and maintainable for production use.

Steps to implement:

1. Declare a native method in your Java class:

   public class NativeMemoryCopier {
       // Load the compiled native library (.so for Linux, .dll for Windows)
       static {
           System.loadLibrary("NativeCopier");
       }
   
       /**
        * Copies data from a native char* to a Java byte[]
        * @param ptr Address of the native char*
        * @param charCount Number of char elements to copy
        * @param dest Target byte array (must be large enough to hold all bytes)
        */
       private native void copyCharPtrToByteArray(long ptr, int charCount, byte[] dest);
   }
   

2. Write the corresponding C implementation:

   #include <jni.h>
   #include <string.h>
   
   JNIEXPORT void JNICALL Java_NativeMemoryCopier_copyCharPtrToByteArray(
       JNIEnv *env, jobject obj, jlong ptr, jint charCount, jbyteArray dest) {
       // Cast the long pointer to a native char*
       char* nativeCharPtr = (char*)ptr;
       // Get a direct pointer to the Java byte array
       jbyte* destBytes = (*env)->GetByteArrayElements(env, dest, NULL);
       if (destBytes == NULL) {
           return; // Handle memory allocation failure
       }
       // Copy the data: adjust sizeof(char) if using wide chars (e.g., wchar_t)
       memcpy(destBytes, nativeCharPtr, charCount * sizeof(char));
       // Release the byte array back to the JVM
       (*env)->ReleaseByteArrayElements(env, dest, destBytes, 0);
   }
   

3. Compile the C code into a native library and call the method from Java.

Critical Caveats to Avoid Disaster #

• Pointer Validity: Never access a pointer that's been freed, points to memory outside your process, or is uninitialized—this will crash the JVM instantly.
• Character Encoding: C's char is ambiguous: it can be 1-byte (ASCII/UTF-8) or 2-byte (wide chars). You must know the exact encoding/byte size of the native data to avoid garbage in your byte[].
• Byte Order: If working with wide chars, note that native systems often use little-endian byte order, while Java uses big-endian. You may need to swap bytes if the data is UTF-16.
• Memory Management: If you allocated the native char* yourself, ensure you have a native method to free that memory to avoid leaks.
• Unsafe Risks: Unsafe bypasses Java's memory safety guarantees—any mistake here can corrupt memory or crash the JVM. Stick to JNI for production systems.

内容的提问来源于stack exchange，提问作者Monseigneur Artous