摘要：是平時相當常用的實現其中的實現比較直接有時候也使用把元素插入到指定的上在中的實現是略有差別需要保證當前數組容量夠用然后把從處一直到尾部的數組元素都向后挪一位最后把要插入的元素賦給數組的處一直以來我都認為這個方法它的實現是調用底層的直接方便

ArrayList是平時相當常用的List實現, 其中boolean add(E e) 的實現比較直接:

/**
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return true (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

有時候也使用 void add(int index, E element) 把元素插入到指定的index上. 在JDK中的實現是:

/**
 * Inserts the specified element at the specified position in this
 * list. Shifts the element currently at that position (if any) and
 * any subsequent elements to the right (adds one to their indices).
 *
 * @param index index at which the specified element is to be inserted
 * @param element element to be inserted
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public void add(int index, E element) {
    rangeCheckForAdd(index);

    ensureCapacityInternal(size + 1);  // Increments modCount!!
    System.arraycopy(elementData, index, elementData, index + 1,
                     size - index);
    elementData[index] = element;
    size++;
}

略有差別, 需要保證當前elementData 數組容量夠用, 然后把從index處一直到尾部的數組元素都向后挪一位. 最后把要插入的元素賦給數組的index處.

一直以來, 我都認為 System.arraycopy 這個native方法, 它的c++實現是調用底層的memcpy, 直接方便, 效率也沒問題.

但今天看了openJDK的源碼發(fā)現并非如此.

以openJDK8u60 為例, 在 objArrayKlass.cpp 中:

void ObjArrayKlass::copy_array(arrayOop s, int src_pos, arrayOop d,
                               int dst_pos, int length, TRAPS) {
  assert(s->is_objArray(), "must be obj array");

  if (!d->is_objArray()) {
    THROW(vmSymbols::java_lang_ArrayStoreException());
  }

  // Check is all offsets and lengths are non negative
  if (src_pos < 0 || dst_pos < 0 || length < 0) {
    THROW(vmSymbols::java_lang_ArrayIndexOutOfBoundsException());
  }
  // Check if the ranges are valid
  if  ( (((unsigned int) length + (unsigned int) src_pos) > (unsigned int) s->length())
     || (((unsigned int) length + (unsigned int) dst_pos) > (unsigned int) d->length()) ) {
    THROW(vmSymbols::java_lang_ArrayIndexOutOfBoundsException());
  }

  // Special case. Boundary cases must be checked first
  // This allows the following call: copy_array(s, s.length(), d.length(), 0).
  // This is correct, since the position is supposed to be an "in between point", i.e., s.length(),
  // points to the right of the last element.
  if (length==0) {
    return;
  }
  if (UseCompressedOops) {
    narrowOop* const src = objArrayOop(s)->obj_at_addr(src_pos);
    narrowOop* const dst = objArrayOop(d)->obj_at_addr(dst_pos);
    do_copy(s, src, d, dst, length, CHECK);
  } else {
    oop* const src = objArrayOop(s)->obj_at_addr(src_pos);
    oop* const dst = objArrayOop(d)->obj_at_addr(dst_pos);
    do_copy (s, src, d, dst, length, CHECK);
  }
}

可以看到copy_array在做了各種檢查之后, 最終copy的部分在do_copy方法中, 而這個方法實現如下:

// Either oop or narrowOop depending on UseCompressedOops.
template  void ObjArrayKlass::do_copy(arrayOop s, T* src,
                               arrayOop d, T* dst, int length, TRAPS) {

  BarrierSet* bs = Universe::heap()->barrier_set();
  // For performance reasons, we assume we are that the write barrier we
  // are using has optimized modes for arrays of references.  At least one
  // of the asserts below will fail if this is not the case.
  assert(bs->has_write_ref_array_opt(), "Barrier set must have ref array opt");
  assert(bs->has_write_ref_array_pre_opt(), "For pre-barrier as well.");

  if (s == d) {
    // since source and destination are equal we do not need conversion checks.
    assert(length > 0, "sanity check");
    bs->write_ref_array_pre(dst, length);
    Copy::conjoint_oops_atomic(src, dst, length);
  } else {
    // We have to make sure all elements conform to the destination array
    Klass* bound = ObjArrayKlass::cast(d->klass())->element_klass();
    Klass* stype = ObjArrayKlass::cast(s->klass())->element_klass();
    if (stype == bound || stype->is_subtype_of(bound)) {
      // elements are guaranteed to be subtypes, so no check necessary
      bs->write_ref_array_pre(dst, length);
      Copy::conjoint_oops_atomic(src, dst, length);
    } else {
      // slow case: need individual subtype checks
      // note: don"t use obj_at_put below because it includes a redundant store check
      T* from = src;
      T* end = from + length;
      for (T* p = dst; from < end; from++, p++) {
        // XXX this is going to be slow.
        T element = *from;
        // even slower now
        bool element_is_null = oopDesc::is_null(element);
        oop new_val = element_is_null ? oop(NULL)
                                      : oopDesc::decode_heap_oop_not_null(element);
        if (element_is_null ||
            (new_val->klass())->is_subtype_of(bound)) {
          bs->write_ref_field_pre(p, new_val);
          *p = element;
        } else {
          // We must do a barrier to cover the partial copy.
          const size_t pd = pointer_delta(p, dst, (size_t)heapOopSize);
          // pointer delta is scaled to number of elements (length field in
          // objArrayOop) which we assume is 32 bit.
          assert(pd == (size_t)(int)pd, "length field overflow");
          bs->write_ref_array((HeapWord*)dst, pd);
          THROW(vmSymbols::java_lang_ArrayStoreException());
          return;
        }
      }
    }
  }
  bs->write_ref_array((HeapWord*)dst, length);
}

可以看到, 在設定了heap barrier之后, 元素是在for循環(huán)中被一個個挪動的. 做的工作比我想象的要多.

如果有m個元素, 按照給定位置, 使用ArrayList.add(int,E)逐個插入到一個長度為n的ArrayList中, 復雜度應當是O(m*n), 或者O(m*(m+n)), 所以, 如果m和n都不小的話, 效率確實是不高的.

效率高一些的方法是, 建立m+n長度的數組或ArrayList, 在給定位置賦值該m個要插入的元素, 其他位置依次賦值原n長度List的元素. 這樣時間復雜度應當是O(m+n).

還有, 在前面的實現中, 我們可以看到有對ensureCapacityInternal(int) 的調用. 這個保證數組容量的實現主要在:

/**
 * Increases the capacity to ensure that it can hold at least the
 * number of elements specified by the minimum capacity argument.
 *
 * @param minCapacity the desired minimum capacity
 */
private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity);
}

大家知道由于效率原因, ArrayList容量增長不是正好按照要求的容量minCapacity來設計的, 新容量計算的主要邏輯是: 如果要求容量比當前容量的1.5倍大, 就按照要求容量重新分配空間; 否則按當前容量1.5倍增加. 當然不能超出Integer.MAX_VALUE了. oldCapacity + (oldCapacity >> 1) 實際就是當前容量1.5倍, 等同于(int) (oldCapacity * 1.5), 但因這段不涉及浮點運算只是移位, 顯然效率高不少.

所以如果ArrayList一個一個add元素的話, 容量是在不夠的時候1.5倍增長的. 關于1.5這個數字, 或許是覺得2倍增長太快了吧. 也或許有實驗數據的驗證支撐.

關于這段代碼中出現的Arrays.copyOf這個方法, 實現的是重新分配一段數組, 把elementData賦值給新分配的空間, 如果新分配的空間大, 則后面賦值null, 如果分配空間比當前數組小則截斷. 底層還是調用的System.arraycopy.