看过SoftReference类的java源码，是不是会疑惑clock的值是怎么来的？
找找C++源码。。。。。。(jdk版本: jdk9-b66)
hotspot/src/share/vm/memory/referenceProcessor.cpp
void ReferenceProcessor::init_statics() {
// We need a monotonically non-decreasing time in ms but
// os::javaTimeMillis() does not guarantee monotonicity.
jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
// Initialize the soft ref timestamp clock.
_soft_ref_timestamp_clock = now;
// Also update the soft ref clock in j.l.r.SoftReference
java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
_always_clear_soft_ref_policy = new AlwaysClearPolicy();
_default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
NOT_COMPILER2(LRUCurrentHeapPolicy());
if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
vm_exit_during_initialization("Could not allocate reference policy object");
}
guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
RefDiscoveryPolicy == ReferentBasedDiscovery,
"Unrecognized RefDiscoveryPolicy");
}
此处将clock设置为当前时间。
此文件中有个函数process_discovered_references
ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc,
AbstractRefProcTaskExecutor* task_executor,
GCTimer* gc_timer,
GCId gc_id) {
NOT_PRODUCT(verify_ok_to_handle_reflists());
assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
// Stop treating discovered references specially.
disable_discovery();
// If discovery was concurrent, someone could have modified
// the value of the static field in the j.l.r.SoftReference
// class that holds the soft reference timestamp clock using
// reflection or Unsafe between when discovery was enabled and
// now. Unconditionally update the static field in ReferenceProcessor
// here so that we use the new value during processing of the
// discovered soft refs.
_soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
bool trace_time = PrintGCDetails && PrintReferenceGC;
// Soft references
size_t soft_count = 0;
{
GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
soft_count =
process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
is_alive, keep_alive, complete_gc, task_executor);
}
update_soft_ref_master_clock();
// Weak references
size_t weak_count = 0;
{
GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
weak_count =
process_discovered_reflist(_discoveredWeakRefs, NULL, true,
is_alive, keep_alive, complete_gc, task_executor);
}
// Final references
size_t final_count = 0;
{
GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
final_count =
process_discovered_reflist(_discoveredFinalRefs, NULL, false,
is_alive, keep_alive, complete_gc, task_executor);
}
// Phantom references
size_t phantom_count = 0;
{
GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
phantom_count =
process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
is_alive, keep_alive, complete_gc, task_executor);
// Process cleaners, but include them in phantom statistics. We expect
// Cleaner references to be temporary, and don't want to deal with
// possible incompatibilities arising from making it more visible.
phantom_count +=
process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
is_alive, keep_alive, complete_gc, task_executor);
}
// Weak global JNI references. It would make more sense (semantically) to
// traverse these simultaneously with the regular weak references above, but
// that is not how the JDK1.2 specification is. See #4126360. Native code can
// thus use JNI weak references to circumvent the phantom references and
// resurrect a "post-mortem" object.
{
GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
if (task_executor != NULL) {
task_executor->set_single_threaded_mode();
}
process_phaseJNI(is_alive, keep_alive, complete_gc);
}
return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
}
然后一步一步的执行到了 policy->should_clear_reference
Java提供了几种SoftReference策略
这四种策略在文件jdk9/hotspot/src/share/vm/memory/referencePolicy.hpp中定义(代码这里就不贴了)
前两种没必要研究
3，4 在文件jdk9/hotspot/src/share/vm/memory/referencePolicy.cpp中
#include "precompiled.hpp"
#include "classfile/javaClasses.hpp"
#include "memory/referencePolicy.hpp"
#include "memory/universe.hpp"
#include "runtime/arguments.hpp"
#include "runtime/globals.hpp"
LRUCurrentHeapPolicy::LRUCurrentHeapPolicy() {
setup();
}
// Capture state (of-the-VM) information needed to evaluate the policy
void LRUCurrentHeapPolicy::setup() {
_max_interval = (Universe::get_heap_free_at_last_gc() / M) * SoftRefLRUPolicyMSPerMB;
assert(_max_interval >= 0,"Sanity check");
}
// The oop passed in is the SoftReference object, and not
// the object the SoftReference points to.
bool LRUCurrentHeapPolicy::should_clear_reference(oop p,
jlong timestamp_clock) {
jlong interval = timestamp_clock - java_lang_ref_SoftReference::timestamp(p);
assert(interval >= 0, "Sanity check");
// The interval will be zero if the ref was accessed since the last scavenge/gc.
if(interval <= _max_interval) {
return false;
}
return true;
}
/////////////////////// MaxHeap //////////////////////
LRUMaxHeapPolicy::LRUMaxHeapPolicy() {
setup();
}
// Capture state (of-the-VM) information needed to evaluate the policy
void LRUMaxHeapPolicy::setup() {
size_t max_heap = MaxHeapSize;
max_heap -= Universe::get_heap_used_at_last_gc();
max_heap /= M;
_max_interval = max_heap * SoftRefLRUPolicyMSPerMB;
assert(_max_interval >= 0,"Sanity check");
}
// The oop passed in is the SoftReference object, and not
// the object the SoftReference points to.
bool LRUMaxHeapPolicy::should_clear_reference(oop p,
jlong timestamp_clock) {
jlong interval = timestamp_clock - java_lang_ref_SoftReference::timestamp(p);
assert(interval >= 0, "Sanity check");
// The interval will be zero if the ref was accessed since the last scavenge/gc.
if(interval <= _max_interval) {
return false;
}
return true;
}
仔细看看发现
【下面写了一堆 还是删掉了，直接看代码吧，都能懂】
总结下：
存活时间大于_max_interval就回收
就是3，4策略对于_max_interval的计算方式不同
这里涉及到一个配置项 -XX:SoftRefLRUPolicyMSPerMB=1000
LRUCurrentHeapPolicy:当前堆可用空间，每兆1000毫秒
LRUMaxHeapPolicy: (当前堆大小-已使用的)
_heap_capacity_at_last_gc - _heap_used_at_last_gc
LRUMaxHeapPolicy: (最大堆大小-已使用的)
MaxHeapSize - _heap_used_at_last_gc
-----------------------------------------------如有错误请指正------------------------------