Java数组的长度到底能有多大

在确保内存大小的前提下，使用Oracle的Java VM，以下代码肯定会报错：

int[] max = new int[Integer.MAX_VALUE];

错误信息是：

引用

java.lang.OutOfMemoryError: Requested array size exceeds VM limit

注意这里的错误信息不是“java.lang.OutOfMemoryError: Java heap space”，意思是申请的数组大小已经超过堆大小。详细参考官方说明： http://docs.oracle.com/javase/7/docs/webnotes/tsg/TSG-VM/html/memleaks.html#gbyvi。根据 Java语言规范，数组下标可以使用0 到 Integer.MAX_VALUE (=0x7fffffff) ，可以使用不代表一定能够使用，要看OS、Java VM能生成到多大。

从 https://jdk7.java.net/source.html下载Java VM源代码 openjdk-7u40-fcs-src-b43-26_aug_2013.zip

\openjdk\hotspot\src\share\vm\oops\arrayKlass.cpp（150）:

objArrayOop arrayKlass::allocate_arrayArray(int n, int length, TRAPS) {
  if (length < 0) {
    THROW_0(vmSymbols::java_lang_NegativeArraySizeException());
  }
  if (length > arrayOopDesc::max_array_length(T_ARRAY)) {
    report_java_out_of_memory("Requested array size exceeds VM limit");
    JvmtiExport::post_array_size_exhausted();
    THROW_OOP_0(Universe::out_of_memory_error_array_size());
  }
  // ......
}

\openjdk\hotspot\src\share\vm\oops\arrayOop.hpp(109):

// Return the maximum length of an array of BasicType.  The length can passed
// to typeArrayOop::object_size(scale, length, header_size) without causing an
// overflow. We also need to make sure that this will not overflow a size_t on
// 32 bit platforms when we convert it to a byte size.
static int32_t max_array_length(BasicType type) {
  assert(type >= 0 && type < T_CONFLICT, "wrong type");
  assert(type2aelembytes(type) != 0, "wrong type");

  const size_t max_element_words_per_size_t =
    align_size_down((SIZE_MAX/HeapWordSize - header_size(type)), MinObjAlignment);
  const size_t max_elements_per_size_t =
    HeapWordSize * max_element_words_per_size_t / type2aelembytes(type);
  if ((size_t)max_jint < max_elements_per_size_t) {
    // It should be ok to return max_jint here, but parts of the code
    // (CollectedHeap, Klass::oop_oop_iterate(), and more) uses an int for
    // passing around the size (in words) of an object. So, we need to avoid
    // overflowing an int when we add the header. See CRs 4718400 and 7110613.
    return align_size_down(max_jint - header_size(type), MinObjAlignment);
  }
  return (int32_t)max_elements_per_size_t;
}

\openjdk\jdk\src\share\native\common\sizecalc.h(41):

#include  /* SIZE_MAX for C99+ */
/* http://stackoverflow.com/questions/3472311/what-is-a-portable-method-to-find-the-maximum-value-of-size-t */
#ifndef SIZE_MAX
#define SIZE_MAX ((size_t)-1)
#endif

32位系统中size_t是4字节的，在64位系统中，size_t是8字节的。

Java 7 Update 40（Windows）测试结果是：

引用

32bit的Java VM： 0x3fffffff - 3 (= 1,073,741,820)
64bit的Java VM： 0x7fffffff - 2 (= 2,147,483,645)