ArrayList
是平时相当常用的List
实现, 其中boolean add(E e)
的实现比较直接:
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
有时候也使用 void add(int index, E element)
把元素插入到指定的index
上. 在JDK中的实现是:
/**
* 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++;
}
略有差别, 需要保证当前elementData
数组容量够用, 然后把从index
处一直到尾部的数组元素都向后挪一位. 最后把要插入的元素赋给数组的index
处.
一直以来, 我都认为 System.arraycopy
这个native方法, 它的c++实现是调用底层的memcpy
, 直接方便, 效率也没问题.
但今天看了openJDK的源码发现并非如此.
以openJDK8u60 为例, 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<narrowOop>(src_pos);
narrowOop* const dst = objArrayOop(d)->obj_at_addr<narrowOop>(dst_pos);
do_copy<narrowOop>(s, src, d, dst, length, CHECK);
} else {
oop* const src = objArrayOop(s)->obj_at_addr<oop>(src_pos);
oop* const dst = objArrayOop(d)->obj_at_addr<oop>(dst_pos);
do_copy<oop> (s, src, d, dst, length, CHECK);
}
}
可以看到copy_array
在做了各种检查之后, 最终copy的部分在do_copy
方法中, 而这个方法实现如下:
// Either oop or narrowOop depending on UseCompressedOops.
template <class T> 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, 在设定了heap barrier之后, 元素是在for循环中被一个个挪动的. 做的工作比我想象的要多.
如果有m个元素, 按照给定位置, 使用ArrayList.add(int,E)
逐个插入到一个长度为n的ArrayList中, 复杂度应当是O(m*n)
, 或者O(m*(m+n))
, 所以, 如果m和n都不小的话, 效率确实是不高的.
效率高一些的方法是, 建立m+n长度的数组或ArrayList, 在给定位置赋值该m个要插入的元素, 其他位置依次赋值原n长度List的元素. 这样时间复杂度应当是O(m+n)
.
还有, 在前面的实现中, 我们可以看到有对ensureCapacityInternal(int)
的调用. 这个保证数组容量的实现主要在:
/**
* 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容量增长不是正好按照要求的容量minCapacity
来设计的, 新容量计算的主要逻辑是: 如果要求容量比当前容量的1.5倍大, 就按照要求容量重新分配空间; 否则按当前容量1.5倍增加. 当然不能超出Integer.MAX_VALUE
了. oldCapacity + (oldCapacity >> 1)
实际就是当前容量1.5倍, 等同于(int) (oldCapacity * 1.5)
, 但因这段不涉及浮点运算只是移位, 显然效率高不少.
所以如果ArrayList一个一个add元素的话, 容量是在不够的时候1.5倍增长的. 关于1.5这个数字, 或许是觉得2倍增长太快了吧. 也或许有实验数据的验证支撑.
关于这段代码中出现的Arrays.copyOf
这个方法, 实现的是重新分配一段数组, 把elementData
赋值给新分配的空间, 如果新分配的空间大, 则后面赋值null, 如果分配空间比当前数组小则截断. 底层还是调用的System.arraycopy
.
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