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 true (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 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.templatevoid 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.