c465ee8cb5
Change-Id: Ibd45b235c9547dd83b27fedbd414fa5c5eb6a42a Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3781549 Reviewed-by: Samuel Huang <huangs@chromium.org> Auto-Submit: Sam Maier <smaier@chromium.org> Commit-Queue: Samuel Huang <huangs@chromium.org> Cr-Commit-Position: refs/heads/main@{#1027416}
81 lines
2.8 KiB
Markdown
81 lines
2.8 KiB
Markdown
# Java Asserts in Chromium
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This doc exists to explain how asserts in Java are enabled and disabled by
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Chromium's build system.
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## javac Assertion Bytecode
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Whenever javac compiles a Java class, assertions are transformed into the
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following bytecode:
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```
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Code:
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0: getstatic #2 // Static field $assertionsDisabled
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3: ifne 20 // Conditional jump past assertion throw
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12: new #3 // Class java/lang/AssertionError
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19: athrow // Throwing AssertionError
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20: return
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// NOTE: this static block was made just to check the desiredAssertionStatus.
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// There was no static block on the class before javac created one.
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static {};
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Code:
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2: invokevirtual #6 // Method java/lang/Class.desiredAssertionStatus()
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5: ifne 12
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8: iconst_1
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9: goto 13
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12: iconst_0
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13: putstatic #2 // Static field $assertionsDisabled
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16: return
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```
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TL;DR - every single assertion is gated behind a `assertionDisabled` flag check,
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which is a static field that can be set by the JRE's
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`setDefaultAssertionStatus`, `setPackageAssertionStatus`, and
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`setClassAssertionStatus` methods.
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## Assertion Enabling/Disabling
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Our tools which consume javac output, namely R8 and D8, each have flags which
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the build system uses to enable or disable asserts. We control this with the
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`enable_java_asserts` gn arg. It does this by deleting the gating check on
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`assertionsDisabled` when enabling, and by eliminating any reference to the
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assert when disabling.
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```java
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// Example equivalents of:
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a = foo();
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assert a != 0;
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return a;
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// Traditional, unoptimized javac output.
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a = foo();
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if (!assertionsDisabled && a == 0) {
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throw new AssertionError();
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}
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return a;
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// Optimized with assertions enabled.
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a = foo();
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if (a == 0) {
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throw new AssertionError();
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}
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return a;
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// Optimized with assertions disabled.
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a = foo();
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return a;
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```
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## Assertion Enabling on Canary
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Recently we [enabled
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asserts](https://chromium-review.googlesource.com/c/chromium/src/+/3307087) on
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Canary. It spiked our crash rate, and it was decided to not do this again, as
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it's bad user experience to crash the app incessantly for non-fatal issues.
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So, we asked the R8 team for a feature which would rewrite the bytecode of these
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assertions, which they implemented for us. Now, instead of just turning it on
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and throwing an `AssertionError`, [R8 would call a provided assertion
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handler](https://r8.googlesource.com/r8/+/aefe7bc18a7ce19f3e9c6dac0bedf6d182bbe142/src/main/java/com/android/tools/r8/ParseFlagInfoImpl.java#124)
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with the `AssertionError`. We then wrote a [silent assertion
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reporter](https://chromium-review.googlesource.com/c/chromium/src/+/3746261)
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and this reports Java `AssertionErrors` to our crash server without crashing
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the browser.
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