Android Jni : crash in global and local ref variables

问题

I have jni c++ code that calls java objects methods, example :

jclass JIOManager = CJavaEnv::getInstance()->env()->FindClass(ioManagerName);

    ..... some code

CJavaEnv::getInstance()->env()->DeleteLocalRef(JIOManager);

this works really fine when executes in main thread, but when I try to execute this in another thread it crash because it says that it is created in another thread :O !! but after I read in google android developer website, they say accessing Global Ref from any thread is fine, so I changed that code to be like.

jclass JIOManager = (jclass)CJavaEnv::getInstance()->env()->NewGlobalRef( CJavaEnv::getInstance()->env()->FindClass(ioManagerName) );

    ..... some code

CJavaEnv::getInstance()->env()->DeleteGlobalRef(JIOManager);

but I get this crash :

09-21 15:38:13.259: E/dalvikvm(3210): JNI ERROR (app bug): local reference table overflow (max=512)
09-21 15:38:13.259: W/dalvikvm(3210): JNI local reference table (0x76fb50) dump:
09-21 15:38:13.259: W/dalvikvm(3210):   Last 10 entries (of 512):
09-21 15:38:13.259: W/dalvikvm(3210):       511: 0x40a609a8 java.lang.Class<libcore.icu.NativeConverter>
09-21 15:38:13.259: W/dalvikvm(3210):       510: 0x42349610 byte[] (1 elements)
09-21 15:38:13.259: W/dalvikvm(3210):       509: 0x41d866b8 java.lang.Class<com.Cocoa.CocoaAndroidSDK.CocoaUnicodeConverter>
09-21 15:38:13.259: W/dalvikvm(3210):       508: 0x423495f8 byte[] (2 elements)
09-21 15:38:13.259: W/dalvikvm(3210):       507: 0x42349428 byte[] (1 elements)
09-21 15:38:13.259: W/dalvikvm(3210):       506: 0x41d866b8 java.lang.Class<com.Cocoa.CocoaAndroidSDK.CocoaUnicodeConverter>
09-21 15:38:13.259: W/dalvikvm(3210):       505: 0x42349410 byte[] (2 elements)
09-21 15:38:13.259: W/dalvikvm(3210):       504: 0x422b3f08 byte[] (1 elements)
09-21 15:38:13.259: W/dalvikvm(3210):       503: 0x41d866b8 java.lang.Class<com.Cocoa.CocoaAndroidSDK.CocoaUnicodeConverter>
09-21 15:38:13.259: W/dalvikvm(3210):       502: 0x422b3ef0 byte[] (2 elements)
09-21 15:38:13.259: W/dalvikvm(3210):   Summary:
09-21 15:38:13.259: W/dalvikvm(3210):       172 of java.lang.Class (3 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         1 of java.lang.String
09-21 15:38:13.259: W/dalvikvm(3210):         8 of byte[] (8 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        20 of byte[] (1 elements) (20 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        27 of byte[] (2 elements) (27 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         2 of byte[] (3 elements) (2 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        16 of byte[] (4 elements) (16 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        40 of byte[] (5 elements) (40 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         2 of byte[] (6 elements) (2 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         4 of byte[] (7 elements) (4 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        22 of byte[] (8 elements) (22 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        15 of byte[] (9 elements) (15 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        40 of byte[] (10 elements) (40 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         3 of byte[] (11 elements) (3 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        17 of byte[] (12 elements) (17 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         7 of byte[] (14 elements) (7 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         8 of byte[] (15 elements) (8 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        15 of byte[] (16 elements) (15 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         8 of byte[] (17 elements) (8 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        16 of byte[] (18 elements) (16 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         3 of byte[] (21 elements) (3 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         3 of byte[] (22 elements) (3 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):        17 of byte[] (24 elements) (17 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         4 of byte[] (28 elements) (4 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         8 of byte[] (30 elements) (8 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         1 of byte[] (32 elements)
09-21 15:38:13.259: W/dalvikvm(3210):         4 of byte[] (33 elements) (4 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         8 of byte[] (34 elements) (8 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         5 of byte[] (36 elements) (5 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         2 of byte[] (41 elements) (2 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         3 of byte[] (42 elements) (3 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         1 of byte[] (56 elements)
09-21 15:38:13.259: W/dalvikvm(3210):         4 of byte[] (66 elements) (4 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         4 of byte[] (72 elements) (4 unique instances)
09-21 15:38:13.259: W/dalvikvm(3210):         2 of byte[] (82 elements) (2 unique instances)
09-21 15:38:13.259: E/dalvikvm(3210): Failed adding to JNI local ref table (has 512 entries)
09-21 15:38:13.259: I/dalvikvm(3210): "AsyncTask #2" prio=5 tid=16 RUNNABLE
09-21 15:38:13.259: I/dalvikvm(3210):   | group="main" sCount=0 dsCount=0 obj=0x41decfd8 self=0x776200
09-21 15:38:13.259: I/dalvikvm(3210):   | sysTid=3236 nice=10 sched=0/0 cgrp=bg_non_interactive handle=7541280
09-21 15:38:13.259: I/dalvikvm(3210):   | schedstat=( 0 0 0 ) utm=82 stm=70 core=1
09-21 15:38:13.259: I/dalvikvm(3210):   at libcore.icu.NativeConverter.openConverter(Native Method)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.nio.charset.CharsetEncoderICU.newInstance(CharsetEncoderICU.java:71)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.nio.charset.CharsetICU.newEncoder(CharsetICU.java:27)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.nio.charset.Charset.encode(Charset.java:451)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.lang.String.getBytes(String.java:870)
09-21 15:38:13.259: I/dalvikvm(3210):   at com.Cocoa.CocoaAndroidSDK.CocoaUnicodeConverter.UTF8_TO_UTF16(CocoaUnicodeConverter.java:16)
09-21 15:38:13.259: I/dalvikvm(3210):   at com.Cocoa.CocoaAndroidSDK.CocoaSDKNativeLib.load(Native Method)
09-21 15:38:13.259: I/dalvikvm(3210):   at com.Cocoa.CocoaViewer.MainActivity$LongOperation.doInBackground(MainActivity.java:1498)
09-21 15:38:13.259: I/dalvikvm(3210):   at com.Cocoa.CocoaViewer.MainActivity$LongOperation.doInBackground(MainActivity.java:1)
09-21 15:38:13.259: I/dalvikvm(3210):   at android.os.AsyncTask$2.call(AsyncTask.java:264)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.util.concurrent.FutureTask.run(FutureTask.java:137)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
09-21 15:38:13.259: I/dalvikvm(3210):   at java.lang.Thread.run(Thread.java:856)
09-21 15:38:13.259: E/dalvikvm(3210): VM aborting
09-21 15:38:13.259: A/libc(3210): Fatal signal 11 (SIGSEGV) at 0xdeadd00d (code=1)

Any help is appreciated.

回答1:

The "other" thread must also be attached to use the JIOManager. Why not get a local reference also there? On the other hand, your crash dump suggests that actually you are only looking at 3 classes. So You can create the global references to these once (even in JNI_OnLoad() if you find it easier), and use them from all your native threads.

回答2:

There is a limit on the number of local references per thread, and you're exceeding it. The most common way to do this is to create local references in a loop and not delete them as you go.

Local references are cleaned up automatically (and near-instantly) when execution returns to the VM, so for simple uses its best to just let the VM do the work. But if you're creating more than 16 local references, it's a good idea to do the clean up manually.

The call to FindClass creates a local reference. The call to NewGlobalRef creates a global reference, without destroying the local reference. You should modify your code to look like this:

jclass temp = CJavaEnv::getInstance()->env()->FindClass(ioManagerName);
jclass JIOManager = (jclass)CJavaEnv::getInstance()->env()->NewGlobalRef(temp);
CJavaEnv::getInstance()->env()->DeleteLocalRef(temp);

Global references are not destroyed unless you explicitly destroy them, so be careful about leaking them. When CheckJNI is enabled there's a cap of a few thousand; without CheckJNI they're allowed to accumulate without bound.